You should have V brakes, the side pull brakes wouldn't fit even if you had the mounting holes.
With most V brakes there should be two adjusting screws on the arms themselves, these screws bear directly on the springs, tightening one whilst loosening the opposite should move the brakes across nicely.
Even on a relatively heavy e-bike it should still be possible to set V brakes so as to throw you over the bars when used aggressively, so you shouldn't see much loss of stopping power.
People always think that heavy bikes stop slower than light ones, but as weight increases so does the friction with the road, were more friction helps to slow you down and counteracts the extra momentum....bhlaa bhlaa bhlaa..., it works mathematically.
Fit the rim brakes you've bid on (they need proper setting up) and you will stop!
A cheap option if you miss out is to buy a supply of good soft brake blocks, once you get the hang of setting them you shouldn't have any problems.
