Like most careers, that depends on your goals, expectations and ambitions plus a lot of luck, I think. It can be fairly lucrative once you build a customer base, and especially if you can buy out a functional business from someone who is retiring or some such, and pick up a franchise or warranty work contract or two. I assume you DO know what "small engine" means, right? It's not just the 3.5 HP lawn mower engines; it's much more than that and for the right people can be pretty interesting work. There are several different types of "small engines".
Some things to think about: Work for yourself? A major company? A sales/rental place? A factory repair center? Etc. When we lived in Chicago, it was a pretty good business to be in. Here in far northern NY it's not so much since the farms are falling like flies and most people aren't into tractors and things like that anymore. With the recent changes in engine design I suspect it can take some doing to keep a good license and continuing education credits and all that. The few such businesses that are around here do very well, but that's because the chaff has been separated out and only the quality folks are left, which is a good thing for the customer, not so good for the business if they like to cut corners or have no people powers.
IMO it is subject to downturns in the economy and such, but what isn't? Heck, even NASA people fear for their jobs today! The trick would be to get into a niche market of some sort where the requirement for the experience and abilities was needed whether the economy was good or bad. With the exception of military contracts, that is; IMO, nothing worse than taking a military contract. I say that because I did . Got rich and got poor, all pretty quickly. Gvt work is more feast/famine than most, although they all are to a degree these days. The trick is to be savvy, grab opportunities, keep up your ambition and love what you do. Find a job you love and you'll never work a day in your life but you'll play every day and get paid for it.
HTH, Pop