I currently run an outdoor power equipment shop where I constantly have to tell people their equipment isn’t worth repairing. They rarely want to hear it but spending 2/3 of the price of a new piece of equipment on repairs is a bad investment since a new one will be new and under warranty.
Of course racing motors are different but a stock World Formula is pretty close to a riding mower in this respect. Now, if the OP wanted to build a modified motor and already had a bunch of parts it would be a different story but since the stated goal is to have a reliable (RELIABLE) recreational motor I’m pretty confident saying his best bet is replacement, not repair.
The process of doing the rebuild is definitely worthwhile from a learning perspective but doing it right could easily require several tries. I learned a whole lot working with Mike Clements, and it’s possible that this particular engine just needs a new piston, rings, rod, crank, and the valves lapped but without experience it’s gonna be tough to know what can be reused and what should be trashed.
The best solution of course would be to get another kart with a WF already on it and rebuild this one so he has two packages….
Would be interested in knowing what he decides, it helps everyone on here when the OP reports back on what they learned in the real world.