I don't collect but enjoy them at antique engine and tractor shows. There are running model versions I've kicked around buying from a place called Sterling engines I think it is.
I have a few. I have a 50 hp Bolinder twin cylinder crude oil hot bulb engine that used to power a sawmill in Michigan and I have a Lister CS 6 hp single diesel. I also have an old Fairbanks Morse inline 6 diesel engine that powered a fire sprinkler system for the University of Iowa. It’s not a flywheel engine though. I used to have a Witte MD 12 electric power plant (their description, not an actual plant) that used to power a creamery In Minnesota. I sold that recently to free up some space. I really regret doing that now though. I figure if I’m ever able to retire I’ll start dragging them to some shows. I enjoy going to them when I get the chance. I may be going to more as I have recently given up motorcycles due to an accident and likely won’t ride anymore. Riding was my main hobby for the last 40 years. Time to move on I reckon.
Messed with a few as a kid at my local county fair. A local family had a fantastic collection and would run hour-long shows several times a day throughout fair week. Every spare moment I had was spent with that group that week.