Re: First build : Need a bike to build on.
In an emergency stop, I grab the front brake (not clutch), close the throttle and the stalled engine helps me stop. The stalling motor (2 stroke) does not lock the wheel, but acts similar to a jake brake on an 18 wheeler.
I agree that you can use that ATB frame if the engine will fit within the front triangle.
You can use an old thumb shifter for a throttle and continue using the grip shift.
(And get a kill switch from a powersports shop) You can also get a thumb throttle at the powersports shop.
Alternatively, since this is your first build, take off the grip shift for a while and pedal single speed, using the limit screws on the derailleur to keep the chain on a cog that will give you a 2 to 1 gear ratio (32 teeth in front, 16 teeth in back, for example).
“You're only young once, but you can be immature as long as you like" -Unknown