I fix up project karts like that all the time and race them too. You can get a WKA rule book for abut $35 to give you the specs and you could race it in the gold cup div with that briggs flat head. You could do dirt oval with a BSP Clone motor or put a Briggs LO206 on it and race asphalt road course.
Just take it all apart, clean everything, weld any cracks and do mods to the frame, touch up the paint with a $2 rattle can from Walmart, replace any bearings or other components that are shot, new brake seals and braided steel brake lines, put it back together with new nuts and bolts, drill and cotter pin all bolts on steering and brake systems. Get a new set of body work and 4 Bridgestone YKC tires and drive it like you stole it.