You can't just leave the engine and not pay the bill or you will never see it again, they will repair it and lien sale it to themselves.
The average mechanic rate here is about $100/hr so you are not necessarily getting ripped off in that regard. I'll pile on here though, if you didn't think the guy knew what he was doing you shouldn't have let him take the engine.
FWIW, there are several things that would cause the engine to POP. Assuming it was not mechanical, a lean condition caused by gummed up carb jets, exhaust valves being out of adjustment, exhaust leak etc. My vote would be for a gummy carb if it ran well when you last used it. Spraying carb cleaner into the throat of the carb to clean jets is about as effective as pouring drain cleaner into a manhole cover to clear the sink drain in your kitchen. If plugged, the carb needs to be disassembled and cleaned.
An engine will run with very low compression and you can't always diagnose that with a pull on the cord if the engine uses an automatic decompression system for starting. Somebody more familiar with your engine may be able to advise you about that.
Bottom line, if you have no confidence in the mechanic get the engine back before they can rack up big fees. Or let them fix it and pay up.