Any kart shop, or just about anywhere for your first question. Fun karts by nature use sort of rudimentary off-the-hardware-store-shelf items. I have a set of Briggs Raptor throttle kits (may or may not be rudimentary) that have a lot of useful parts in them, bought off fleabay.
We have a parts supplier thread in the FAQ that should be mostly up to date.
MFG Supply Good for basic karts parts Harbor Freight Engine The Infamous Harbor Freight engine M&D Mower Replacement parts for the Clone-Hondas and Hondas. Also great for manuals and other replacement parts for all sorts of engines. Affordable Go Karts Great site for aftermarket parts and...
www.diygokarts.com
I have no experience with the Tecumseh OHV but all these stationary industrial engines use a “Rube Goldberg” contraption, I mean governor linkage that’s pretty complex. There’s no direct linkage to the carb on the vast majority of these engines like you said. The governor is a rev maintainer so that’s paramount on a tiller. Lots of people keep the governor intact….