I would use a timing chain and gear set instead of a belt then.
No slippage at all, ever! No adjustments. Should last forever at such low RPM and low stress.
I think last set I bought for a 350 chevy was around $25 and came with the cam/crank gears. Might have cost less even, been awhile since I bought one.
One VERY nice thing about timing chains is that I have never seen one BREAK in normall use. Yes I have replaced MANY broking timing belts (some almost new) but never have I seen a broken timing chain. Well, I have but that was a frozen cam or something on a race engine that busted it, not normal.
Yes after enough wear they do jump time, but we don't have to worry about that with a genie since we don't care about valves and pistons both being in an exact spot at a certain time. All we care about is that both are turning, no worries about them matching up. So even a well used chain set would be fine for this type app.
I think chains also loose less power than belts, that makes startup easier.