An engine that always brings a smile to my face is a twin cylinder Mamod SE-3. They are fairly common and also tend to be quite reasonable in price. I think there are a few on evilbay currently at $150 or less, and a couple of them look pretty good.
You might want to check some out .... here's one that caught my eye:
https://www.ebay.com/itm/195970877698?It is listed as a BIN without a make offer button. But I find that if you contact the seller with a reasonable offer of maybe only 10-15% off, it is sometimes accepted!
Also, for real cheap entertainment, it is hard to go wrong with the Mamod Minor overtypes. Some of the best little table top dancers money can by, and it isn't unusual to score one well under $100.
See here:
https://www.ebay.com/itm/285644792838?