A GPS can be a very good addition instead of trying to make a 60+ year old speedometer work accurately and reliably. If you're still running a 6 volt electrical system, a small 12 volt motorcycle battery will power a GPS and a pretty good entertainment system for several days between recharge sessions.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!