As to physical limitations there aren't any. A 900cc engine will bolt right in the frames are exactly the same. Your biggest issues might be electrical and cost. There isnt any reason a 750 won't last as long as a 900. Change the oil.
should something go "Bang" a used 900ss engine will probably be cheaper than repairing the 750. electrics are identical between similar generation bikes. meaning there won't be problems going from 750 carb to 900 carb or 750 injected to 900 injected. BUT going from carb'd to injected will kick your butt unless the injected engine comes with every electrical thing on the bike.
i'm also worried about going from anything to 800, as the 800 has the latest and greatest of stuff. here, too, you'll probably need "everything" to make this happen. THAT is likely to get spendy.
I do change oil religiously so I hope to get the most of out of the engine. I guess I'm just thinking down the road.
I'm also itching for a bit more power now. I'm not regreting the 750 purchase since it was my first "big" bike so it was great to ease into, but after about a year now I'm addicted and want more! [smiley=evil.gif].
I guess I'm thinking of a cost effective way of killing 2 birds with one stone and so a 900ss swap sounds very appealing when it's time .
Just out of curiosity why'd you recomend a SS engine instead of a Monster 900 of a same year?
I'll jump in and answer the SS v. M engine question since i think i know..
the SS has different cams that gives you more power up top.
having an old 900SS and a new M900 in the garage, I can verify this with my highly tuned and calibrated Ass-Dyno.
the Monster feels a little stronger on the bottom, but runs out of steam way before the SS.
I wouldnt sweat the mileage if you're willing to do the maintainence. my SS has 45,000 and has no problems. it's ready for some engine work...like a piston/ring kit and valve guides/seals. not a big deal.
if you like the idea of hopping the 750 up, there's lots of parts available.
if my wife would let me hop up her M750, and i had a buncha money, i would do:
hi-comp pistons, 39mm flatslide carbs, dyna coils, light flywheel, and maybe have some headwork .
of course, it may be cheaper to get a bigger bike or engine, just depends if you're looking for final result, or if you like the tinkering process.