does anyone know the merc part number or a good alternative

Go to mercruiserparts.com pick out a 97'-98' 500 , got to oil filter adapter.
It doesn't list a complete setup, but you can
piece together what you need.

Keith Eickert makes a a real nice oil filter/thermostat I think they call it a Thermofilter. They also offer just the oil thermostat that you can plumb inline.

Billet Marine has a nice unit.

merc makes a conversion kit ,it has everything you need to convert an older engine to the new style oil thermostat system.call me and i can give you a price on it,315 623 7642

GO WITH THE KEITH EICKERT AWSOME SET UP