One youtube guy I really learn a lot from calls himself "mustie1".
He explains his thinking and how he gets some of the really nasty stuff running (sometimes not). He shows a lot of the ways of checking out and getting 'stuff' back into running order (and then he sells the thing from his front yard in New England).
He started off with a LOT of small engine stuff like lawnmowers and to break up the monotony he branched off in a couple of other directions.
Antique engines that haven't run in decades, Old VWs (he used to be a VW mechanic), all sorts of mechanical stuff including chain saws, boat motors and if you can imagine it he has (usually) gotten them up and running.
A lot of basic stuff - you know - spark, fuel, air, compression explaining to you what he's looking for. He usually puts up a video every Sunday morning - and sometimes if the project is bigger he'll make multiple videos.
There are other channels that I follow but his is the hands down best.
Lets see if I can pull up a good'un. Here is an OLD snow blower to start you off on:
https://www.youtube.com/watch?v=6VXK7DCZVaQBe careful - once you start watching these .........................