First you need to determine why the engine is frozen, it may just be from sitting, or it may have greanded at sometime in the past and parked. Being a two stroke it's pretty easy to work on. First pull the heads and have a look, it what you see looks normal use a some from on penetrating oil, I like P.B. Blaster these days, squirt it down and let it sit for a few days, then try see if you can get it to break free. Once you have that pull the barrels and have a good close look, change the rings out ans hone the barrels before you put it back together.

Also check the reeds and transfer (crank case) seals, clean everything out and try to get it to come alive. Be certain that the injection pump is working, I might mix oil directly with the gas at a 50:1 ratio when you first start it to check tat the oil pump is working.

If you find it's a mess (holed/burned piston or seizure) you know what to do...

John.