Only place I found the OS for sale:
https://www.rcmart.com/o-s-engine-o-...1d201-00105510
Also looks like most not TOTL racing engines are only for sale from HK right now, at least I didn't find any anywhere else. Here are two options on eBay. I think the prices are more than fair, especially since you're not looking to race and they're already including a pipe. Tempted to pick one up as a spare for my dad's MTX2 even:
https://www.ebay.com/itm/Gimar-3PL-2...MAAOSwJ3hf-dJG
https://www.ebay.com/itm/Novarossi-N...sAAOSwn4VbjXPY
And here's the shop behind those eBay listings:
Engine
Get an engine heater too, best case even get the engine EBIS'd to get more life out of the conrod, who knows how available parts for these engines will be down the line.