73' 40hp merc 402 powerhead rebuild?

[lazzlonewolf]

I bought a 40hp merc that a guy ran hot, I checked the compression and got 50psi and 60psi in the other. I know thats low. Now i have another motor that reads 120 in both cycs. Now can i just swap the pistons out or do the cycs need to be bored? Oh the other motor broke a crankshaft or id just use it. any better ideas? I really dont want to pay for new pistons and rings if my old ones will work.