Your 85 was probably running a 2.33:1 gear ratio. Your 120 is probably running a 2:1 ratio. Difference is prop shaft is turning 17% faster. The HP difference is 120/85 = 41%. So your thrust effect is 41-17 = 24%. Increasing your pitch by 24% would put you in a 21P.
I'm running a similar rig with a 20P SS Mercury Laser !! at 5250 RPM lightly loaded and get stellar performance. You can expect the hull to ride higher in the water with your same load so you will get more performance per inch of prop pitch, same style prop, with this higher HP. So it's sort of a crap shoot. If you do most of your running at/near WOT, and want a guess at an appropriate prop, in a low cost aluminum in the 13+ diameter, and for a hundred buck crap shoot, a 3 blade General purpose aluminum this ought to work for you:
http://boatpropellers.iboats.com/Fo...130-21_Propeller/194/37959/?cart_id=428920861
If you do a lot of mid range running, like cruising, at lower speeds and want better control of the boat attitude...speed stays constant just over planing speed, doesn't drop back and increasing throttle causes it to run away requiring throttle to be cut back etc. etc. then this one:
http://boatpropellers.iboats.com/Fo...130-19_Propeller/194/37958/?cart_id=428920861
They don't offer a 20P in this prop. The iboats listing goes no higher than 1996, but the 1996 lower unit should be the latest LU design having the through hub exhaust, not the snout your 85 probably has, and the 15 tooth spline.
You furnished no performance information which would significantly help in answering your question specifically, but what I offered should/will work. Question is, will you be happy with it.
