Bushing bearing or ball bearing version? (Fox made both if I remember correctly...)

Ball bearing isn't as bad to replace the bearing as bushing type. Either can be done... but the bushing type is such a hassle you usually just junk the engine.

The pressure in the engine case gets higher at high throttle than at idle... so leaking more at high throlttle is common when the bearing is starting to fail. The next symtom will be an unstable idle or inability to idel slow enough to not have the model sit in place without holding it. While its just leaking and not bothering idle... you just have a bit of a fuel loss.

You can't pull the crankshaft without pullig the piston. If its a sleved engne, you have to pull the cylinder sleeve to get the piston out... which means you will be dissassembling EVERYTHING. It is best to get some in-person assistance with this the first time you do it... unless you are willing to trash the engine if you goof.