No power, engine skips fire?

if you are losing compression, you will usually see some oil seepage near the head gasket or sometimes along the seam where the two halves of the cases join

If it is due to a bad crank seal, then oil may show up under the rotor or behind the small bevel gear

a weak CDI can also make that popping noise

It could be ancompression loss at the rings if the bike was babied too much during break in... what happens is the cylinder walls will glaze before the rings seat so the bike will run good during break in and it'll gain some power as things free up, then it'll begin to lose power. On a 4 stroke engine you would see it start to smoke during acceleration and it would get progressively worse before any power loss happens, but on a 2 stroke the engine feels strong one day then it'll feel not so strong but it'll still run good, then a few days later it'll start to get boggy and may die if trying to go full throttle under load. As it gets worse it'll bog just reving the engine.
if you got a compression tester try running a compression check to see how much compression pressure you get.
if its a head gasket issue, there will be some oily residue at the failure point but if its the rings you wont see anything but a perfectly shiny cylinder. A new set of rings and a re hone will get ya back up and running, but if you don't have a 48mm flex hone its probably cheaper to replace the cylinder too

Sounds like a compression leak. Start the engine, disengage clutch and lock it with the button, see if you can keep it going with the throttle and hold your other hand close to all sides of the head and see if you feel any hot air shooting out, often times compression leaks happen at the head.

All the answers are definitely possibilities... Best thing to do is pull the clutch and mag cover to check for fuel residue which would point to a case leak (easy) and then do a compression check (also easy, but need a compression tester).
Another test you could perform would be to seal off the intake and exhaust ports (those rubber stoppers for lab testing work really well here, just need one with a hole in it so pressure can be applied by a hand pump or small compressor, or tire inflator etc) and pressurize the case to about 10 psi and use some soapy water on all the seams to see if you can find bubbles showing the leak... One thing that works really good for finding a case leak is a bottle of kids bubbles, just pour a little over the suspected area once pressure is applied and it'll start blowing bubbles at the leak.
A little bubble soap around the head and turning over the engine will reveal even a small leak at the gasket.

Sounds good to me... Glad to hear you got it up and running again

Thanks for the update!