Chris, the easiest thing is just a simple metal (or plastic) plate/disc that you can rotate over the hole. I've know of guys mounting a weed whacker metal choke plate (Ryobi's used that for years, I think), then actuating the tap from the front of the cowl. You just reach in there and push it over the hole when needed. You can make it fancy or plain, but anything will do, heck almost penny soldered to a pushrod lever would work.

Differences in where the carb needs to sense the pulse from the crankcase could explain it. Many Walbros do not have external pressure pulse fittings, and have holes in the base that have to align to the spacer and holes from crankcase. I wouldn't be concerned.