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?
Yeah I’ve been burnt badly on eBay myself on other things.. and not the biggest Mamod fan. They seem junky to me, but that’s my opinion.