If it has good compression and runs then why not just run it as is? Just change the oil, new plug, clean carb and put fresh gas in it and ride the hell out of the minibike. I wouldn't get any small engine bored, honed, new piston and bearings because it cost too much. I can buy great running 7hp Tecumseh for about $70 or a newer 10hp for about $100 in my area.