stupid engine is making rust, now it wont run properly and keeps pumpin gunk into the fuel tank

so i finally decided to take my kyosho gx12 apart. inside as i slowly remove the parts there was heeps of brown liquid everyhwere! inside i saw the main culprit is the ball bearings, thy are creating heeps of rust, after wahing out all the parts with WD-40, the only rust left is the one which has been stained to the walls of the crank case and con-rod.

how can i thoroughly clean the engine ? can i soak the whole crank case in some gasoline ?
what will clean the rust stains

after that how do i put in new bearings ? i know how to remove them.
i'll only put new bearings if my local rc shop has replacement ones

ive been considering replacing my whole engine. but with a different one ofcourse. i wanted to know if any engine within the .12 or .15 engine will fit the kyosho pureten gp alpha ?

or what other engines will fit in there ? hopefully someone can point me to a cheap used one as i am low in cash.

thanx to evry1 who can help