You can't easily replace the frame though. You're right that the other parts can be replaced, but you should have at least used a good frame to start with. For only a few hundred bucks more you could have gotten a substantially better bike to work with. The bike you have has a cheap aluminum frame, which will accumulate stress fractures over time from the high speed electric bicycle use exceed its design specs that it's seeing. It may eventually catastrophically fail on you.

At your given budget I would have used a better bike and a smaller motor/battery. The combination of beefy motor on an inferior quality bike could be flat-out dangerous.