I used to work with motor starters and O/L's quite frequently (I'm a partial commercial HVAC service tech, and partial in-office paper pusher), and I still have to locate and purchase replacement parts for them every now and then, but I'm not an electrician, so take this with a grain of salt:
Looks to me like you have two options: Pop for a replacement O/L relay (there are still some new ones available online), or replace the entire motor starter. The former keeps you in "old" technology ("stuck in the 70's..."), while the latter
may offer an "upgrade".
https://www.radwell.com/Buy/SIEMENS/FURNAS ELECTRIC CO/48EH18A6S2
While neither option is inexpensive, this
is a safety device you're talking about....
Of course if you happen to "know a guy...who knows a guy...who knows an electrician", you may be able to swing some sort a deal.
Good luck!