I take it your Briggs is a 60102? If so, I don't understand how you'd have an output shaft size problem - the ones I've seen step up in size from 5/8" only in the last 2 mm before the oil seal on the crankcase cover. Can you post a picture of the output shaft for us?
However if you are happy with the Kirby, there does not seem to be any problem in using it. The major mechanical aspects of them seem very durable.
