how hard is it to put a honda engine in a yamaha dirtbike?

i have an older 250 2-stroke honda engine and ive found an 80's yz125 dirtbike with a bad engine, so what i was wondering is how hard would it be to put the engine in that bike and what modifications, parts, ect. i will need to do the engine/frame to swap the honda engine into the yamaha frame... anything you can tell me will be a big help, thanx in adv.

If you have a cutting tool and a welder, you can do anything, i put a 75 yamaha on a 78 honda

I put a Suzuki in a Yamaha