+1 on buying a new engine. Used nitro engines are almost always trashed. I've gotten lucky a couple times and burned many more times. I even bought one used engine that was so badly worn the crankshaft wobbled in the crankcase. The seller tried to tell me I damaged the engine by opening it to inspect it. No, I definitely did not smear aluminum all over the inside of the crankcase where the crankshaft counterweight was dragging against it, just by opening up the engine. Fortunately eBay sided with me and I got my money back.
I suggest running 30% nitro fuel if you can get it. The extra nitromethane content makes the engine less susceptible to tuning problems caused by changes in the weather, because a greater percentage of the oxygen needed to burn the methanol will come from the nitromethane instead of from the air. It makes learning to tune a nitro engine much less tedious.
Also, there is a limit to how much power a nitro engine can safely produce. You can't just keep leaning-out the fuel mixture until you get the power you want; you will oil-starve the engine and wear it out very quickly. If the engine you have can't produce the power you want without running so lean the exhaust stream has no visible smoke, then you need to buy a bigger or better engine.