You can scroll through here and see what others have used.
Howdy, MEP-802a and MEP-803a use all the same filters and such MEP-802A ONAN DN2M 2-cylinder engine (Lister-Petter LPW2) Liquid cooled diesel, aluminum housing MEP-803A ONAN DN4M 4-cylinder engine (Lister-Petter LPW4) Liquid cooled diesel, aluminum housing Oil Filter Spin-On (FILTER ELEMENT...
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The thing to watch out for is the gen is 24V but 80% of the gages out there are 12V.
Try to find a 24V 80 PSI gage, to make it easier.
Beede makes a lot of Marine gages which are affordable and look close to original, but you also have to buy a little $14 adapter board with a resistor in it to use with your 24V system.
I think your sender is ok. Think you had 240 ohms at 0 Psi. That sounds correct off the top of my head.
240 - 33 ohms if I recall correctly.
If you want to change it, I believe any 80 PSI sender with a 1/8" NPT thread and 240 -33 range will work.
I have used Sierra and Datcon before.