Power issues

i would start at the carb adjust your idle screw on the left side of the carb run it in all the way and turn it out 3 and a half turns. if this helps pull in your clutch and set the idle to where you want it

if your leaking oil its from your exhuast gasket if there not installed just right they heat up and melt and burn off and oil will leak all over your motor i recomend you get the thickest exhuast gasket you can and put on there i made my own out of high temp gasket material and put 3 stacked on top of each other and never had a problem but it was thinner gasket material no power could indicate that you dont have back pressure on your pipe and a leaking gasket would do that also try taking out your plug and seeing if its fouled out if so buy a new plug see if it helps try putting your thumb over the spark plug hole to check for compression pop the clutch and keep it locked and rotate the engine over while pressing down at the spark plug hole there should be lots of pressure there pushing against your finger if it was me i would suspect the carb spark plug cdi or magneto all of these are cheap and easy to replace

I've seen main jets come out of the factory VERY rich - a new engine will also run rich till broken in. Try moving the needle down to the last notch for a while till it runs in a bit more.

note: one moves the clip up to drop the needle down

Make sure you tighten everything. If you're getting leaks near the plug and the exhaust, you're losing pressure/compression. That'll make any motor seem gutless.

You said it's a used motor? What fuel mix are you running? If it's already been broken in by someone else you would get less power running at break-in mix levels.

Main jet has either fallen into fuel bowl as has been said or main jet is way to big which I have found to bethe case on all china gorl engines I have owned, sound like a good chance that its jst running very rich or you could have an exhaust that is semi plugged up.

Hard to pin it down without actually hearing it run but at least you have some things to check into.

Map

If it revs fine with the clutch pulled in its most likely your muffler is clogged. I had the same problem

I have the same issue only my bike won't rev or idle. I've had the engine for 3 day and is the 3rd engine I've bought. (The others was stolen from me). And when I pull the clutch in the bike shuts off. I've seen oil on the muffler and the spark plug. Pls. Help me