As Topas mentioned pay attention to the design and detail of the overall cooling system. For points specific to the VW engine: Make sure it has a good valve job with minimum slop on the valve guides and adequate seat/valve contact area. While this isn't technically a cooling issue it does make sure that all of the heat from the exhaust valve (the critical part) has at least a fighting chance to get passed to the head. Take a look at the cooling shrouding on a '71 or newer bug engine and try to emulate the air flow around and through the head. Make sure all of the casting flash is filed or cut away at the center line of the head where the mold parting line is. Some castings are actually completely closed in some areas. Next find Veeduber's HVX mods and make sure that all are included, especially a full flow oil filter/cooler and the valve lifter modification. These make such a big difference in cooling and longevity that I quit offering a warranty on my ground bound engines unless they were included. VW makes a fine aircraft engine, but to fracture a quote 'A man's got to know his engines limitations'.