If the problem is actually the key switch and you want to find an OEM style, then do the "farmer-fix" I did under the same situation with my 57 6400 grain truck. I simply used a 110v ON-OFF flip switch. On... or off... until I found a suitable replacement. Just wire all your "switch on" stuff together. ( ON-OFF-ON, if you need separate ACC function.)
Jethro.
ps. Use the same toggle position you use for your lights in the house... up is on, down is off. Saves battery and points. I know.

~~ Jethro
1954 3100
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In the Dity Gallery
1951 3100 (gone) / 1956 4400 (still in the neighborhood) / 1957 6400 with dump body (retired) / 1959 3100 panel (in the woods junked) / 1978 Custom Deluxe K10 / 1993 S-10 4.3 / 2004 Chevy Crew Cab / 1945 John Deere H / 1952 John Deere B / 1966 John Deere 2510 / 1967 John Deere 1020