25 ltr gm motor change

i am new to i/o boats. i recnt

i am new to i/o boats. i recntly purchased a 1985 19ft imperial with a bad motor. the # 3 cylinder has a chunk missing from the piston and damage to the wall. my question is do i have to replace it with a marine motor or can i use an automotive motor? will a mercruiser straight 6 bolt in place of the 4 cylinder; and will the lower drive handle the mercruiser straight 6? any info reguarding this would be greatly appreciated.

"The Merc six will fit OK and

"The Merc six will fit OK and bolt up to your transom assembly; but your front mounts will likely need to be relocated.
You probably have a 1.98 ratio Alpha One drive, which will handle the power, but you will likely have to go to a prop with more pitch. A better ratio for six would be 1.84 or even 1.62, but a change of prop will likely compensate fairly well.
There are quite a few differences between your engine and accessories, and an auto engine, not the least of which is the cam. Maybe you could get the shot cylinder sleeved? I would check with an engine rebuild shop.
Rod"

"Bob:
Before you spend anot

"Bob:
Before you spend another dime, check the transom, stringers and floor for rot--expensive repair."

The rest of the boat is in ver

The rest of the boat is in very good shape. With the exception of the seats. I think i am going to go with the 6 cyl option. Thanks for all your help.

"Think the least expensive rou

"Think the least expensive route would be to get an automotive 2.5 or a short/long block, swap cams, manifolds and all accessories and be back in business without having to think about the new manifolds, motor mounts, starter, alternator, engine coupler, carb and other marine parts you'll need for the 6cyl."

"[b]"Think the least expen

""Think the least expensive route would be to get an automotive 2.5"

Dave,......
The 2.5l motors used in the Chevy automotive Applications is a Totally Different Casting,+ will Not interchange with the Chevy 2.5l Marine motors........"