Welcome to the forum. Here's some reading for you regarding engine mounting in bikes with oversized or curved downtubes. http://motorbicycling.com/showthread.php?t=6427
As for as tires and rims; there's no reason new tires won't work on your rims as long as the rims are in good shape, not rusted and are true. If they need new spokes or truing your local bike shop can help you or there are many tutorials available to help you do it yourself.
Good luck, have fun.