You would need contacts in the US spare parts industry to have much hope of getting Tecumseh valves with oversized stems, I think. When Tecumseh's engine factory shut down there was talk about speculators buying the last stock of spare parts so they could run up the prices.
If the valve guides are pressed in you could probably get a local workshop to turn up a pair of new ones for you. Alternatively you could just find a pair of valves from some other engine that happen to have a slightly larger stem diameter, the same head diameter, and a greater stem length, then shorten the stems and ream out your old valve guides. It would most likely be quicker and easier, though, to just send a PM to Joe Carroll.
