Ugh, If you really must redo the engine and it is glued together well, I would suggest ordering the parts for the engine from the company who makes that model. For example; if Tamiya makes it, call them up and order and sheet of parts that contains the pieces for the engine. I don't know of any way to unassemble plastic that has been glued. Is there no way that you can paint the engine without disassembling it?