your really not going to get a decent return in power for money.

As said, get a decent air filter/panel filter, exhaust, maybe lower it a bit to stiffen it up, and sort the handling. Give it a good service with the correct plugs and oil.

It's not going to be a rocket no matter what, so make it look nice, sound good and handle well and be happy, and keep an eye on the oil level, they can drink it and sieze without you realising