ORIGINAL: Barber420
What brand of fuel are you using? Do you have stock needle setting specs to start from? Have you tried any other kinds of glow plugs? O.S. #8's and A3's always work well for me. I'm guessing the engine that appears to have been abused is probably junk. A nitro engine in the wrong hands can last as short as break in and be junk. If you're tight with cash you could see what a piston and sleeve for the engine costs, but I find it's usually more economical to buy a whole new engine in most cases.
generally ya it is more justifyable to buy a new engine... the piston and sleeve sets generally run in the ball park of half the cost of the engine, while the other costly part is usually the carb and crankshaft...
I would just save up a hundred or so bucks and get yourself an OS engine they are pretty low cost for the .15 & .18 CV series, and are pretty reliable