rebuilding, in the engine world isnt to much, but mower wise, its upwards of $150, kinda expensive, plus you need the right tools, torque wrench, hone, assemboly lube, thread lock, engine oil, ect. I think the 15.5 would be a great motor by itself, and id part out the 20hp on ebay. You'll come out on top from what you paid for. just me. Is it pressure lube (oil filter on side) by chance?