Finally something on singletrack I might have a useful input on!
My anorak hobby is restoring and researching vintage engines, with that I write a bit for Stationary Engine magazine, think Have I Got News For You magazine of the week…..
A good starting point for playing about is a little Suffolk Punch engine off a lawnmower, plenty of spares and cheap to pick up.
Something bigger the Lister D is the stalwart of the engine line up, though prices are increasing for them they are a favorite among collectors (not me..)
If you don’t mind buying from China lots of collectors have had fun with these little hit and miss models. I’m yet to buy one….
https://www.banggood.com/Eachine-ET1-STEM-Upgrade-Hit-and-Miss-Gas-Engine-Stirling-Engine-Model-Combustion-Engine-Collection-p-1430588.html?rmmds=search&cur_warehouse=UK
Another area to look is Stirling/Hot air engines. Great for running inside, you can get simple ones that run on a cup of tea to stuff like the Ryders that will cost you many thousands. There is some nice UK models that done break the bank.
If your near North Wiltshire I might have a little 2 stroke I could pass on to grow the interest/hobby, but I’d rather not see it thrown in the metal recycling bin once the fun is had….