The Sierra might be worth another check. The Crusader number you gave did not cross for me with the Sierra number you gave, so they might be different threads.
With the right adapters or a refit
you could put in anything rated for the application.
Do you want it to be 'stock' or 'easily serviceable' or looking 'racecar'?
Edelmann makes adapters from 5/16 inverted flare to threads or hose barb, 4$ each at Summit Racing
I have no affiliations with anyone listed in this reply, and I am not 'current' with inboards, sold my 1970's I/O years ago...
All the best working through this.