I?m see you have a few options:
Make your own gasket with sheet gasket material like this

https://www.autozone.com/sealants-glues-adhesives-and-tape/gasket-material/p/fel-pro-3157-gasket-material/67683_0_0?spps.s=2201&cmpid=LIA:US:EN:AD:NL:1000000:GEN:19502231967&&CATARGETID=120054150001287771&CADevice=m&gclid=Cj0KCQjwn_OlBhDhARIsAG2y6zNYhFbbrnkLCONmcaf_BOFt_dSv85LmMbz2JUx8ddWlFtOkBUpicuQaAtnQEALw_wcB&gclsrc=aw.ds

Or replace it with a newer one, one that has the proper bypass for jet boats like these:

https://www.cpperformance.com/c-124-thermostat-housings.aspx


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