The 2 speed vacuum shift became obsolete in about 1954 and parts were not available very long after that. I remember some being repaired in the 60's by using an air valve. Spring pressure holds the axle and the speedo adapter in the low range. When vacuum is applied, it pulls the axle into high range. The turning on and of of vacuum can be accomplished with with an air valve. I have one that the speedo adapter leaks vacuum. I have been thinking about using an adapter from an electric shift to shift the speedo, but I haven't really got into it yet.