now don't take this the wrong way, but these motors are not horrible.
they are fine as long as they are well maintained, tuned are they are not starved of oil.
I'm not saying that you did anything wrong, i'm just saying that these are not high quality motorcycle engines...they are toy engines, and they need a little extra attention to make them run a long time.
The cables for instance, could use a good lube now & then with some teflon based lubricant to keep them freed up.
the motors usually seize from lack of lube. the majority of these motors are all aluminum except for maybe the piston sleeve, rings, connecting rod, crank shaft and the gears inside. so saying that there is rust "on every part" is not possible...aluminum will not rust. rust on the chain and sprocket could be from lack of lubrication and them sitting "dry" exposed to humidity. Have you pulled the head / jug off to see why the engine seized up?
I am not sure of your mechanical abilty and / or mechanical knowledge so let me just say that these chain drive kits are not rocket science. (hopefully without insulting you) they are very easy to make work and last IF you know how to set one up right, and tune it correctly. There is no such thing as "bolt on & go" with these kits...there is a lot of tinkering, and modifying to get one set up right.
This is just my opinion, and experience from building / riding 2 different bikes with these motors.