That's an interesting choice of motor / battery for an mtb. The motors are rated at around 5 amps each in normal use, but, and it's a big but, it depends on the propellor size, and hence the current drawn.
a guesstimate for around 10 amps drawn using small sla batteries would be only a few mins of sedentary performance.
If I might make a personal observation based on a lot of models made, the Percasa cries out for a decent brushless motor and lipo battery set up. It would run well, not silly fast ( 'though that is achievable) and it could run much longer and be much lighter in weight.
A basic rule in model boating is that sla batteries are great in ships etc, and darn near useless in anything not designed to crawl along.
Chas