Well, you definitely dont want to run 87/89 octane like in a average 4 stroke. What octane you need depenps on compression. 32:1 is safe mix ratio for gas.
A leakdown test is in order no matter where/who you got it from.
Check ground connections. Make sure key/kill switch are working properly.
New plugs.
You can pull boot off of coil wire and cut off 1/4 inch of wire to expose some clean wire and re install boot.
Set pick up gap.019
Clean stator poles and flywheel
Ohm stator
Swap cdi with a known good one
You need to start over with 27.5 pilot,290 main. 340 main is too big. Set air screws at 1.5 turns out. Move needle to middle clip. Set floats. This is where you should be to start dialing bike in
Prolly want to go ahead and put a 27.5 pilot in. better safe than sorry. Your main sounds close, you gonna have to read up on how to do plug chops to get it dialed in perfect. Nice banshee
we all want to say we built that lil badass, but most successful banshee owners on here will tell you that their first build was a expensive learning curve. Its gets better with second rebuild and so on. haha