please turn off auto correct and use some punctuation...
it's a bit hard to read
that you "buy trade" and looking at "engines too fit"
For your question:
keep in mind that if you stretch the wheelbase as well, you will have to adjust the steering geometry to accomodate.
And that welding on a racing chassis is mostly not too good of an idea if you're not fully aware of how a racing chassis works
(i.e. the flex in the chassis to allow the inner rear wheel to lift in corners for example)
that depends heavily on chassis geometry and axle load.
Any four cylinder bike engine is certainly much heavier than any kart engine,
so at the very least the donor kart should be from a watercooled shifter (ideally a superkart)
since those are actually meant to hit insane speeds and carry a water cooler, pump and a two cylinder twin eingine at least..
it also comes with front brakes, which are more than just a good idea if you want an r6 to propell that deathtrap
'sid