Considering it is costing you nothing (???) as it should NOT cost anything.
If you have the time and patients to rebiuld the boat the go for it.
Fact 1. the lower half of the outdrive (lower unit) is interchangable and has no affect on gear ratio.
Fact 2. your Upper half (upper gear housing) clearly shows 120, 120 means 4 cyclinder motor. as stated the upper gear case gear ratio for a 4 cylinder motor is different from a upper gear case for a V6.
IF you do not have to purchase this boat (as no one should ) then I would say spend all the money and time you want to restoring it.
When you get to the out drive you can run the boat as is, just be prepaired to controll rpms and not go over 4800 rpms.
If you can go past that 4800 the change proppeller pitch and see if you can keep it at 4800 MAX rpms....
The key here is,
No matter what the story behind the boat, it is all of what you are willing to do and SPEND to get it back to operational.
Looks like a very expensive project..........I would sell the motor and transom assembly. and the outdrive (seperately) on craigslist and make money vs spend. take the profit and buy one that runs allready, the difference in price will most likley be less then the cost to fix this one and you will be boating in a clean running boat.
JMHO