My experiences and opinions to get mileage out of one:
First-off, gotta get rear sprocket straight as can be and bolt down the engine so the chain lines up. Next tweak the idler bracket til the chain runs straight on the pulley.
Keep chain under ~3/8-5/8" deflection. I actually got almost 2K miles on the stock kit chain in almost 2 years with this method, the engine had to be cocked in the frame
a lil bit to get a straight chain. I also munched the heck out of my 34T sprocket because I put so many miles on that chain lol
2nd, get a proper killswitch working and make sure you have no intake leaks. The choke should be able to kill a warm motor quickly.
Definitely use allen cap screws with lockwashers on intake and exhaust headers. Blue Loctite or equivalent optional.
3rd run a proper mix and don't overrev the dern thing
Running them more or less out of the box and not revving it much over 6K will net you a torquey engine, from my experience.
Gear it under a 40T and pedal hard til it has a few hundred miles on it
Stock kits with 44T keep it 26MPH or less during break-in, IMHO. If it shakes a lot at 26, check mounts. If mounts are OK you got a shaker. That's a Monday/Friday engine.
If at all the top end starts knocking, you've over-stressed the upper conrod bearing. Rebuild the top end/warranty and start over lol
When it's properly broke in, it will be very susceptible to mods and should last a decent while, depending on the day of the week your engine was built