Questions about 1976 mercruiser i/o 4 cyl 120 hp?

I purchased a boat on ebay with a 120 hp mercruiser motor. Are these decent motors? Also are they made by mercruiser or are they a chevy or ford with mercruiser stickers? Also if they are made by mercruiser will any other motors bolt up to the outdrive unit if I ever have any problems with the motor?