I'm planning on building a classic thunderboat and I was wondering if anybody knows where I can find a free thunderboat plan for a stock Zenoah PUM engine?

No free plans but i can tell you now you will be cheaper buying a kit than cutting your own parts.The Zippkits are cheap and go together easy and can be made to look reasonably scale and best of all they run awesome.Easiest kit you will ever build for this type of boat and probably the best running one too.
Mart
well over 60 mph with a full mod
www.youtube.com/user/martno1fa...u/5/wVGBWC_3MuY

Check to make sure any kit is legal if you intend to race it. Raed the rules first then buy. Harv.

The Blazer Marine Laterbauch is a nice fast thunderboat too.