What are the legal rules on swapping engines?

I know this will vary state by state greatly, but can't even find federal rules none the less for my state (Illinois).

Are you allowed to swap engines on motorcycles legally? I know cars you have basically of the same year or newer, and same "class" (can't legally go putting a truck engine say in a car and so on). Besides obviously modifying the frame, is there anything against say going from Xcc engine to a bigger one? Especially newer when it comes to these older bikes it would be nicer to have a newer and more available engine, and why not bigger.

i have done a lot of swaps and never have had a problem

Some states title a frame, some states title the engine -- that's worth looking into in IL.

Illinois title's the frame. I'm still just worried if I get pulled over, and have a 600cc engine in what was originally a 350cc frame originally. Same with insurance in the unfortunate event of an accident.

If you have a 600cc engine in a bike designed for 350cc, chances are you'll be in a ditch, not being pulled over

I'd be less concerned with police / legality than I would your insurance company. Chances are if you did a major change (like going from 350 to 600cc) and wrecked your bike / body, your insurance company would tell you to go fly a kite.

But, if you go from say 650 to 750 (as I did with my XS650), or from 750 to 900 on a BMW Airhead, that's a potentially different story. My 1975 BMW R75 (750cc) is no different than the R90 (900cc). The only difference in the bikes are the piston diameter and carbs. Frame, suspension, brakes etc. are all the same from 600-1000cc displacement on those bikes (with other subtle differences that wouldn't effect safety).

People have been boring out their cylinders in cars and bikes for decades - don't sweat it.

In PA, as long as you inform the insurance company and they agree to cover it your golden. BUT when it comes to state inspection the shop can be a dick about it if they feel like it.

nc does not even look at the engine number unless the bike is registered to the engine number

It's easier to ask for forgiveness than permission... Words to live by!