I initially want to improve the 750 with a little more power and a 6th gear. But with every year something else becoming unavailable parts wise, I want to not have to worry about parts concerning liquid cooling as there are fewer 906/907 around than the 750.
I'm thinking about how if it's possible to bolt in an air cooled engine from an earlier 900ss. I'm aware of the swing arm and engine mount bolt mismatch, but don't know what years will just bolt in. And those engines are not plentiful either if the case is different than later 900s. Or using a 900 ss cylinders and heads on a 906 crankcase. And is the 907 the same as the 906 but with the added fuel injection? And if the 900ss cylinders fit either crankcase, will the 750 exhaust fit. Or is the exhaust port in a different place to prevent it. I would guess that the 906/907 have larger ID pipes, but I'm not looking for a big power increase. And last I guess a 900 ignition box would be needed. I want to see if it's possible to have a 900 air cooled paso by just bolting on the least number of parts without modifying anything.

Id go for the water cooled 906/7 as pretty much the only thing to wear out is the water pump itself and there the same as the 851, 916 bunch so there wont be an issue parts wise . The plumbing ( inc t/stat ) is a bit unique but there are plenty of ways around this . The 906/7 is taller than the 750 ( longer stroke ) so getting the 750 exhaust to fit the 906/7 would involve a bit of stretching ( not really possible ). You will notice that the water cooling of the 906/7 is only on the cylinder and heads and doesn't actually pump water thru the cases ( apart from the actual pump itself and that is just a side cover thing ). If you were hell bent on an air cooled 904cc donk it aint impossible . the 900ss cases have the needle rollers inside the cases and the spindle rotates in the cases and is clamped to the swing arm, and the paso motor has the bearings in the swing arm and the spindle passes thru the cases ( and both sides of the frame ) but does not rotate , and the SS cases are wider at the pivot point . Really , if you want a 904cc motor just go trade the 750 on a 906/7 , or buy a paso donk ........oh but then youd need a radiator too and the 750 frame doesn't have the mounts for that ..... save yourself a whole bunch of #ucking about (and probly money in the long run) and buy the 906/7.....OR just be happy with the 750 ........heaps of peeps here are more than happy with their 750's and all said and done they are the "original" Paso and should not be scoffed at just because they are 150cc smaller , that power difference can be largely recovered buy a set of Mikuni's off a monster , better exhaust and tweeking the cam timing. There aint nothing consistent about the factory cam timing settings and its not uncommon to find the actual cam timing 8 deg off spec , and even the two cylinders on the same donk timed differently , either offset keys or adjustable cam pulleys is the answer there .

Thanks for the replies guys,
I'm going to start on this project at some point later this year, no talking me out of it. I have ridden my friends 750ss and 900ss. Both are stock internally. The point being the 900 does everything the 750 does with so much less effort, the relaxed pace when sport touring with 6th gear and noticeable torque increase has me sold on it. And the 900 with slower engine speed should have increased longevity.
So I'm looking for the path of least resistance in doing this starting from a 750 Paso, so it's that frame and swing arm. And staying with air cooled.
As already answered about the engine swing arm mounting, would it be easier if I just got a 900 monster or ss and delt with the modification to fit it. Then a set of header pipes and ignition box gets it done? A lot less effort than trying to convert a 906/7 engine. Except they fit right in. My recent searches have not discovered any early 900 engines or cases to bolt right in. The rarity of the early 900s and 906/7 engines keep pointing to the later monster and ss engines. The monster engines are easy to find.
Anyone know what is involved to fit it to a Paso? Any idea about sprocket alignment aside from swing arm issues, are the mount points to the frame the same as the 750 Paso cases?

I don`t think a 900cc engine will live longer than a 750.
Pity the guys who have done something like this are not around. If your P750 has a Kokusan ignition and you`d use an engine from a carbed Monster/SS no changes to wiring or ignition would be necessary. You`d need headers from a 906/907ie as the ones of a Monster/SS would not work due to the Paso swingarm and lower frame rails.
The mounting points to the engine are the same and the bolt to the front cylinder is a larger one on the Paso as far as I remember. No problem to modify.
It seems you could also install a later 6 speed gearbox to the engine (last bulletpoint)
http://www.bikeboy.org/duccapacities.html

did the switch and posted all details about conversion.
An easier way to do this might be find an Elefant 900 engine and install M900//906-907 cams and upgrade to larger valves from earlier M900

that`s a link to your profile

As just said, that's to your profile.
In any event building a semi race engine 750 does not achieve what I want. I'm looking for the added benefit of the 6th gear, (thank you for the info about using the 800 trans conversion) and also more power with factory from new reliability. It's exciting to build a 750 for more performance, convert to an 800 trans, bigger valves, cams, hi comp pistons. But how much more will that cost compared to putting in a 900. And will it even have as much power as a stock 900ss. Or even a monster 900.
Sorry to rant on here.

Oh! I'm very sorry, I fix by now!
The user is lceriani and here's a link to the post where he makes a list of the pieces he needed and the complete job he did:

Thank you Nicola,
I'm going to bore the Paso brethren here to tears occasionally with my Paso story. Soon I will load some images here to begin the saga.