mercury power head swap

[arkieguide]

I have a bad power head on my 7.5 hp mercury. i would like to install a 9.8 in its place. Problem is, I find no serial # on the old block, i have it off on the bench with crank and pistons out can find no serial #, the oem tag is gone. Wire brushed the block no good, So what should I look for ? there are some 6 number groups ?

Thanks
arkieguide