I believe that's a shoulder bolt. "Standard" size ones are available from places like McMaster Carr, if you can determine what the dimensions are. I'd be willing to remove the bolt from my starter to measure it for you if you need dimensions.
[on edit] I pulled mine off and measured it.
The shoulder is 1/2" nominal diameter, 1/4" from head to end of shoulder, and overall length of 1". The threads are 3/8"-18. Has a round head, but anything would work.
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Last edited by klhansen; 07/03/2022 5:49 PM. Reason: Added photo & dimensions

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.