The noise could be from a bad crank bearing or a crank stuffer screw that backed out. The parts are easy to get, as they cross over to honda, yamaha and industrial parts. All Balls has kits too. There were some crossover years so you may want to pull apart the bottom end to see what bearings and seals you need.
I would rebuild the crank, replace the crank bearings, crank seals, waterpump seals, counter shaft seal, shifter seal and kickstarter seal. I would install a new piston and reeds. Whoever pulls apart the engine will tell you if you need any other transmission or powervalve bearings need to be replaced.
The GG 2 stroke engine is very similar to a Honda CR250 from the early 1990s. Any reliable engine builder should be able to do your rebuild for you.
Honestly, if you were able to pull the engine out yourself without a problem, you maybe able to do the rebuild job yourself. Check out this video to get a feel for what is involved.
https://www.youtube.com/watch?v=RS6dtHwED1Y
Halls Cycles in Springfield, IL has a very good supply of parts in stock.
www.Halls-cycles.com
Good Luck