Will a cr500 motor fit into a trx 250r chassis?

Yes it will fit but you'll need to fabricate a lower mount, head stay and the two mounts that bolt to the frame and reach back to the front of the engine case. You need to use an 86 or 87 trx250 frame. (much easier and has a slightly longer stock swingarm than an 88 or 89 trx to keep the front end down). Have a reputable machine shop mill off .200 (two hundred thousands) off the left side of the engine case so the counter sprocket will align to the rear sprocket. Then a spacer will need to be placed in the front swing arm pivot point (the same big bolt that goes through the rear engine case). Loren Duncan of Duncan racing (Southern California) can fab you a kick starter and help you with an exhaust pipe to finish the conversion. NOW,....you'll deal with the vibration! It WILL crack various pipe mounts and brackets on the frame. I know because I've done one! BUT,...It's faster than stink and has enough torque to pull down a tree! The only quad that beat me was a fully built 403cc stroked & ported banshee (also bulit by Loren Duncan).