I ve got a cr85 honda and have just found a small hole in the crank casing on the side of the chain sprocket. Is there anything I can do to repair it or do I need to strip the entire engine and buy a new one?
I would suggest you take it to a good aluminum welder and have it tig welded. Drain the gearbox oil. Remove the sprocket and everything from around the broken area and clean it very good. The welder may be able to weld it without splitting the cases depending upon where the hole is.
You may be able to repair it with JB Weld but you are taking a chance of the JB Weld falling out at the most inopportune time. Best way is to tig weld it.
"good aluminum welder" is the key. Or try the JB weld thing and it might last forever. Or not. The area behind (underneath?) the countershaft sprocket is subjected to abuse due to mud build-up, jammed stones, grass, small branches etc. That is probably why you have a hole? Inspect this area often and keep it clean. Welding is the right way, but the JB weld might actually work (and so might various other epoxies).