I've got one my dad welded out of tubular steel about 20 years ago like that porsche one only wider and coast about £3.

Lie face down and steer by pressing feet into the ground.

There are some mental tracks close to where I live and i'd say conservatively 40mph plus

I went clean through a holly tree on it when i was about 12.......my fookin face wasnt pretty.......altho it isnt much better now