Many of the Glow Engines from the earlier days of RC have been discontinued; yet many of these engines are still in use by modelers today
On the internet you will find several forums for most or all of the current and discontinued RC Engines.
MECOA (Model Engine Corporation of America) – NEW & USED
Rebuilding an engine like Cox, K&B, ENYA, HP and more; it is a good chance that MECOA will have what you need
Glow Engines
Gas Engines
Electric Motors