The KEY part and difference between a cheap kit and a $200 SBP HD kit is the freewheel bearing that isolates your pedal cranks from the engine drive sprocket.
The $20 bearing is OK for a stock 48cc or ~350W electric but shred in short order with anything bigger.
I have seen this first hand twice, one was a 48cc shifter build I did long ago and when his motor died (put regular gas in it) I replaced it with a 66cc.
He didn't get 10 miles before the cheap freewheel went out, and they don't go out friendly.
You can't unscrew the bearing from the special threaded crank arm without a special tool, and that is the part that gets destroyed so there is no way to get the bearing off thus rendering the entire assembly except the inner sprocket you can get off garbage.
Actually I was just noticing yesterday SBP has a new Ultra HD Freewheel Bearing coming soon for you high performance engine guys.
I haven't been able to take out an HD bearing yet but there are some pretty mean engines out there.
One other note is the actual jackshaft housing you attach to the engine.
The cheap shift kits use the old original SBP style 3 part system.
What a pain in the ass in so many ways.
The replacement design is one solid U shaped piece so the actual jackshaft can't wander with mount movement.
Yes they take a lot of adjustments during chain break-in and they are not easy or friendly to make, and sometimes you simply have to use a tensioner.
Another issue is your bikes gears...
Though a 7-speed will work they are far from ideal, and 9 sprockets is just asking for trouble.
Throwing engine torque at 9-speed chain that isn't even in straight line just isn't going to be trouble free, you will miss a shift eventually and you'll snap that chain right in half and without the kits Jump Stop you will find the chain simply won't like first or second gear.
I try to use an internal geared hub whenever I can.
I love the things.
Just one fat sprocket on the rear wheel you can align perfectly with the freewheel bearing sprocket and get tight using fat single speed HD 410 or even HL710 chain.
Bikes have a bunch of close gear ranges to help your measly .2HP legs stay within your body's tiny power band.
Your 2HP engine has a vast power band, enough that you can run them direct drive.
I find a simple Shimano NEXUS 3-speed to be simply ideal.
For a 26" wheel...
2nd gear is pretty close to the gear ratio and speed as a stock direct drive kit with 41-44T sprocket.
1st gear is ~33% gear reduction, sort of like having a 63T rear sprocket for torque.
3rd gear is ~33% gear overdrive, sort of like having a 27T rear sprocket for speed.
All those ratios can differ of course by the hub and sprockets used but you get the point...
You can get going from a dead stop much quicker, and have a faster top speed than direct drive.