Depending upon where you live (are you in the states or Europe or elsewhere?), someone may be able to give you a lead to a modifier (like RCRenew in Charlotte NC) who also repairs them. Years ago, many local hobby shops also had the knowledge and parts pipeline to repair engines.
These days, you are best off being your own mechanic as it is the most viable option.
It also depends on the engine and what is wrong with it, as parts are often difficult if not impossible to find for older engines.
Sometimes buying a replacement engine is your only option.
What engine (the identity) are you dealing with and what appears to the problem?