Just found a vintage Sears & Roebuck 'Ted Williams' Free Spirit 3 wheeler. Can I modernize it?
Sure, anything is possible, especially if you have access to a oxy-gas welding or brazing torch (or arc/MIG/TIG welder), a grinder with a cut-off wheel and a willingness to cause mayhem.

As a BMX, it'll be heavy and ungainly, especially around corners. When you pedal, it'll pull to one side as it doesn't have a differential, but it'll be tons of fun and something different.

Why not put a really big wheel on front and turn it into a rocking trike?