I removed the videos from the channel a while back.
It's funny you should revive this project as I got another razor e300 project on the go, This one is less power but alot more range, I found when you brake the 20mph mark you pay for it heavily in watthours used.
If I was to do a budget build like this one was again I would use a brushless motor from the 48v chinnese scooters only problem is fitting the controller in the battery box there's just no room.
I used the board you mentioned but I would not recommend it the throttle was a pot based one so hall throttle didnt work that meant I had a thumb roller for the throttle and I did have some sketchy times with it and 2 big off's.
If you don't mind a drop in speed then hoverboard wheels make a good cheap build and the generic 36v 350w controllers fit in the razor no problem, could easily have a range of over 20 miles with the space In a razor e300 battery box.
My current build has front and rear hydraulic disc brakes no more rear drum only, the handle bars are more adult like with an ignition in the stem, I swapped the wheels from 4 inch to 6.5 inch with offroad tyres and I've removed the freewheel but kept the 25h chain and geared from 11tooth front and 54 tooth rear or 4.9 - 1 to a 68 tooth rear and 9 toother front or 7.55 -1 its the biggest reduction i can get to fit the frame without the chain line and sprockets interfering with anything, the speed drops from 15mph stock to 10mph but it will wheelie more easily and with the lipo it will be back up to 15mph then.
I've ditched the lead acid and built a lipo battery 8s 24ah that ill use with a 24v 500w controller with an adjustable throttle not on and off like stock and then the stock motor for now, that way I'll have stock speed but alot more torque so it will climb with an adult riding and get good efficency being at high rpm and lower amps doing it.
I may change to a brushless RC motor in time but for now I'm gonna run a brushed set up like In the old wild west.
I forgot loads out ill add so any tips as I remember them any questions I missed fire away.