I agree, you want breaks on both the front and back wheels, coaster breaks are not designed to stop a motorized bicycle. Caliper "V" brakes are fine though, disc is better but not required. I have gone 1000s of miles with my V breaks and haven't even had to replace the pads once yet.
I've been working on a Sixthreezero with a 7-speed in the barrel setup. I remove the shifter and have mounted a 212cc
on it and the bike has more than enough room for a 79cc and even the 212cc "Death-Row bike kit" the mount is a
CNC sent with the kit. The CNC mount should have been longer in length. The mount which would force you to
extend one of the post mounts. I have extended one of the mount (rear seat post mount) and left out two of the
stainless cap screws to keep the engine level. I built a front mount that is attached to the front of the engine which
was originally used for the stock gas tank. This mount is clamped to the front tube of the bike. A channel that is
10 1/2" or 11" long that would replace the original CNC aluminum channel would be the way to go and if a fella
could locate a steel or aluminum channel 7 1/2" x 4 1/8" extended to 11" x 4 1/8" long. The channel would have to
drilled/slotted out for mounting the engine and to use the existing aluminum end mounts would be the way to go.
I managed to tip the engine in the back a few degrees and was able to mount the existing CNC mount but would
rather have the engine sitting level. The GasBike company sells the engine kits and also has the CNC mount that
bolt to the seat tube which has the engine vertically. I might order one to see it it would work on this bike. I wanted
the single speed bike with the instead of the 7-speed that was sent to me. I am not certain that the rear axle bearings
can handle this engine since the axle hub is a 1" size. A single speed has the 1.5" hub and uses a coaster brake
which is good for the 1.5" sprocket hub clamp. You don't have enough sideways movement for clamping to the
hub with the 1" hub. I am seriously thinking of going to mag wheels front and rear with the sprockets and mount.
The V-breaks are ok but a set of disk brakes would be better on this bike. This work is currently in
progress with this bike.