That sounds just like FatJocks did (he let me ride it) before it came apart.
Piston and rod were shot and he posted pics here about 6 months ago.
Might try a search and check it out.
Yup, Mine blew at around 60mph, around 30 miles from home.......when I got it apart, the bottom end and the rod were fine, piston was in "a million little pieces", and the sidewall of the cylinder ripped asunder. The exhaust valves were also bent.
Head was okay, and fortunately the valve seats were fine.
For the new valves, cylinder, piston kit, and gasket kit, was around $600 in parts. That's not counting the additional tools, and service manual, I had to buy to get it done.
Could have got another engine for around $750, and would have taken a lot less work to replace, but I wanted the experience.
I learned a lot from it, and enjoyed doing it, but it took a long time, and a lot of effort.
At the end of the day it's up to you which way to go, but a replacement engine is definitely the simplest.