It can be very rewarding and even lead to many other things and opportunities.
Having a good understanding of cause and effect is valuable in any job, and is learned easily while becoming good at diagnosing engine problems.
As an example,
1. I was an Auto Mechanic for 15 years.
2. That led me to my next career, which was "teaching" auto mechanics, in a trade school for a few years.
3. From which i then started "managing" a phone repair team for Verizon, where I have been for the last 20 years!
So how did i get to #1 above?
For me personally, I started just pumping gas at a station that also did auto repairs.
I talked with the mechanics, helped them on simple jobs, and after a year or so moved from pumping gas to a mechanics helper/aprentice position. Eventually learning and working my way up to a head mechanic.
But another option is to look into various post-high school "Trade Schools". They do cost money, but will give you a quicker path than I took.
If you are still in public school, ask your guidance councilor is there are any local BOCES or Trade "high Schools" in your area as well, thats another option to get started.
Repairing them doesn't necessarily require a college degree but designing them or applying them almost certainly requires a college engineering degree. The engineering degree will most likely earn you a higher salary and access to much more job opportunities.
My first thought is whether a "small engine" means car-sized engines as opposed to large engines in goods vehicles, marine propulsion and generators. In that case, the mechanics' jobs are most likely mostly in the local repair and service garages.
But maybe it means smaller than that? I think the key in that case is to think about where such engines are used and where they'd be taken for repair. I'm thinking a lot of really small engines are in things like lawnmowers, power tools like saws and strimmers, and stuff like that - I'd expect garden centers, tool-hire firms and places like that to have some kind of maintenance and repair facilities available, and I'd imagine those are where you'd want to look for that sort of work.