Mercruiser 3.0l engine excessive blow by but good compression

Hi guys i have a sea ray 175 sport with a mercruiser 3.0l engine it is 1995 model the engine has produced some weird symptoms lately it started with running on which i adjusted timing for now it has produced excessive blow by and yet it runs great it also has issues starting as it sounds like the ring gear has a few teeth bad in one spot and grinds i have done a compression test and have good compression of 3 cyclinders on 125 psi and 1 on 155 psi engine idles well and runs well but is pushing oil out engine breather blow by and starter issues came in one trip out does any one have any ideas? Im a diesel fitter and im stumped.

Ayuh,... Sounds like she's been run hard, 'n put away wet,.... repeatedly...

Ya need to change the ring gear 1st,... 'n yer compression numbers aren't all that great....
Which cylinder had the higher number,..??
Any water in that cylinder,..??

There doesn't appear to be any water in cyclinders an number four had 155psi 3 had 125-130 2 had 125-130 and one had 125-130 i have only just purchased this boat. is the motor changable with a 350 chev it has an alpha one leg or will it be to much horses for the stern to handle if it is good to roll what is typical change over needed