for example, putting a 250cc engine in a 50cc bike, or a 500cc engine in a 250cc bike?
Answers:
I too have been thinking of doing this.
My plan is for a Derbi or Aprilia 50 chassis and a WR250F (OK, now everybody is going to do it). So far it looks like everything will fit but some of the mounting points are not as robust as I'd like. I'm not worried about power output, the 50 frames can handle all sorts of power (relatively) but they can't handle the vibration of the thumper motor. I figure all I need to do is spread the mounting points over a larger area and be sure to add a strong head steady. I also have to figure out a better gas tank, the Derbi has a tiny tank under the rear seat.
There's a guy who stuffed a YZ426F motor in a RS50 chassis. That's overkill and he had some problems.
Other Answers:
No idea booss.. tough question
yes I did and will do again. I put a 125cc in a 50 frame and it did not work out well. the frame was too week for the power. I put a 250 in an enduro frame with some beefing up of the frame and it went on to win several desert races. Next one I want to do is put a Harly sporsted engin in a Suzuki 750 frame that I have modified already and then blew the motor in. the 50cc bike frame will be too weak and u will have to strenghthen it at stress points. better go for an used frame from a bigger bike.
You are very likely going to have to cut & re-weld the frame to accept a bigger engine, which may very well ruin the rigidity of the frame, and cause you rbike to flex. I think you are better off just modifying your existing engine with reasonable mods (K&N filter, upgraded exhaust, re-jetting) or selling it and buying abigger bike. Remember a more powerful engine will also require an updated suspension, and better brakes to handle the extra power.
Source(s):
30+ yrs with bikes