That's basically a Force 9.9 engine. If I had to guess, it looks like an early 80's. I owned a couple of mid 80's Force 15 hp motors that had the same basic layout. For me on a 15hp with a relatively narrow 14' aluminum boat, there was absolutely no issue with planing. A 9.9 with two guys and gear, that could be approaching the limit of that motor. I recently ran a 6 hp on the same boat with me and my 4 year old, and I could just BARELY get it to plane off with him in the front seat.
Have you tried it with just yourself? Also, it may be easier to have your buddy sit in the very front seat until the bow comes down. Also make sure that you have the motor trimmed all the way in.
Sorry I can't be more helpful. My motors were good, although I had one overheat as there was no tell-tale to see if the water was running or not, so keep a very close eye on your water intakes and replace the impeller regularly.
Compatible motors would be: Force, Sea King, Gamefisher, Chrysler, looks like model years between '84 and '87
Good luck!