It's this one:
https://www.treatland.tv/Puch-50cc-Hi-compression-head-p/puch-hi-comp-head-50cc-old.htm
I'll install it tomorrow after work and see what kind of changes it makes.
Nick - exhaust and a bunch of other goodies are in the mail. I was hoping they'd be here in time for the weekend but it looks like they'll get here Monday. (edited)