I've owned both of those machines. I still own an 80's Honda 185 ATC. I don't think it's impossible to switch them out. But I'm pretty sure the outer dimensions of the 250 are bigger. If you have a welder and something to cut with, I think you'll have to make the area for the engine bigger. You can just extend the lower part of the frame by 4 inches or so, by adding a piece of heavy tubing. But I think you'll have to raise the upper part of the frame (over the engine) by a couple of inches as well. If you can measure,cut and weld- it shouldn't be a huge problem. Also, I'd save all the electrics, ignition, etc. from both machines till I had it all put together. Also, you may need the sprockets and chain to be from the same machine (either the fourtrax or the 3-wheeler). I kinda think the fourtrax has a heavier chain and wider sprockets. If you can post some pics, I might be able to help more. I'm just going on memory from several years ago.