Pony engine starting motor "magnetic switch"

[Rftlts]

Hello, My '58 720D's "Pony Motor"'s starting motor's starting relay part number AR 39959 is not working well using 6v. if I put the 12v battery back in the relay will pull just fine. But I am tired of charging the battery. Has anyone repaired these? or know of a good part number/source for a new one? Thanks Loren