I would get a one piece cylinder CG kit, there made by flying horse, there a bit tricky to find but well worth it if you can, there great CG engines, very little vibration (unlike my buddys 2 piece head, that thing feels like a impact wrench)
Only problem ive been fighting recently is the magneto and cdi box, but other than that, its been problem free!
4 stroke kits are also great as a first, just measure the frame as you need a larger frame to fit one, heard they are very reliable, but ive beaten my CG up daily for about a year now (ive even ridden it in snow! Started right up in below 0!)
Good luck! Hope it works out for you!
(Also, below is picture of the one piece head engine, try to look for those in ebay pictures)
EDIT: i saw you went with the pk80, hope that works out for you! If you need help people here are very friendly!