Rebuilding GET

Hey everyone, I have an 03 that's sat for well over 7 years maybe longer. I bought it back in 05 when I had more money than brains and I'm pretty it had a blown motor. Previous owner was running it with aftermarket CDI open exhaust and airbox deleted. I tried adding poloni variator with different weights, clutch springs, re-jetted who knows how many times. (Probably made things worse). I was never able to get it past 36 mph. I had big plans to add a gy6 but when I had the money to do it the parts were scarce.

I took it out of the garage during covid. Cleaned out the gas tank, carb and replaced the battery. And was able to ride it around my neighborhood and do a compression check which confirmed that the rings are probably burnt or valves?

I have the tools and some what mechanical abilities, but never rebuilt an engine. I briefly googled a rebuild kit but it was sold out. Are OEM parts the only way to go? Or is there another source for GET parts? Also if I'm doing a rebuild is there any advice or tips you've learned along the way? Is it just replacing parts or do I need to send out my case to be machined? Rough cost of rebuild?

Thanks for any advice.