This is an old project. I bought an used engine online 5 years ago and went abroad. The engine seems to be in good compression and fires with carb cleaner.

Today I made a fuel reservior which feed fuel directly to carburettor. It started and kept running. Then I started adding coolant through radiator cap while motor was running. I kept pouring 2 gallons of coolant and started wondering why it took so much coolant. I started look online and found out this is way too much coolant. The engine kept running and then the overflow nipple on the valve cover started to spit out oil-coolant mixture... That kinda explains why... So the coolant I added from radiator cap went into the engine and mixed with oil.... The whole time the temp gague didn't move at all, and I touched radiator, it was cold the whole time.
I stopped engine and unplugged the radiator hose, the coolant from the hose is clean, non-contaminated. The tempeature of the coolant is slight warm...

What could be the odds?

Head gasket? Cracked block/head? That don't happen too much on a motorcycle, right?
A bad oil cooler? Oil cooler seems to be a solid compoent, does this happen often?
A bad water pump? The impeller shaft seal is bad and allow coolant to enter???