The OD of the seal you require is larger than the std lower engine case seal of the Kirby, Lauson (eg LAV 30 HK25 VK 30) and some tecumseh engines.

This I suspect is the one you found, as its still readily available.

Im sure I saw your seal on ebay under H30. Alternatively, take it to a seal/ bearing supplier and see if they can match it.

Also check The online store under Tecumseh, it may actually be there.