1991seadoo

Hey everyone im new a this forum stuff...
Any way i got a 1991 sp 580 in the spring for allmost free!

The guy i got it from said it had been rebuilt but beeing a hands on guy i looked at it and yes all new gaskets and top end!

So i put it togethere as it came in pieces.

i cleaned everything up and i fired it up it ran great then took it to lake,
It would only fire on new plugs so i did that twice then it wouldnt start again.

So i took off carb and rebilut it also noticed crankcase full off oil.
took it to a meachnic he said crank seals.....
How do i test the crankcase pressure?
And what other test can i do to get to the bottom of this??

Im looking at buying a newer motor but how do i know this wont happen again..
THANKS TO ALL hope to hear something