Welcome to the forum, what kind of bike do you have?
The motor depends on your wallet.... Many will disagree, but syuff a big china motor in it and call it a day. If it takes a dump on ya, through it away and buy another n for a couple hundred dollars.... my buddy Chris has a ZB with a GPX 140, reliable, and tops out at 75MPH and Strong. Its not $4K Takegawa motor, but gets the job done.
If your going to paint your frame, powder coated or have it sprayed base coat clear coat. Don't waste your time with spray paint... there's always chrome.....
~Shawn