In that case.. yes, sounds like a plan.
The 120bhp has the same gear linkages and driveshaft part numbers, so assuming that the bell-housing fits to the engine itself, the rest of it looks like you can use your existing 115bhp components. I imagine the 120 does not have the 6-speed box however.
The 16V engines are a different kettle of fish. The 136 (Belgium), 140 and 150 engines have different part numbers than the same component on the 115, so they're likely to be more problematic... although not necessarily impossible to adapt. There's no hard and fast rule.. the 136 and 150 are the same in some respects but different in others to the 140, while the 140 and 150 might share parts that don't fit the 136 .. the only common feature is that none of them share parts with the 115.
Obviously the parts might work +/- some 140/150 parts from whoever sells you the gearbox but you might have to swap too many parts to make it worth your while, unless it's just a hobby and it doesn't matter if you can't make it work. The best example is the front wishbone arms.. the later cars are stamped rather than cast items and they can only fit to the earlier cars if you also fit the front sub-frame. It's possible... but maybe not the simple swap you imagined.
Unless someone has done it and knows differently, obviously..
Meanwhile... pop the gear cables off the ball/knobs on the gearbox selector and make sure they both move freely. The rubber gaitor will be long since perished and the cables point slightly uphill, so collect "splash" which can drain into the cable. Stick some grease down the cables (lithium is best since it won't dry out and turn stodgy) and see if the inner cables move freely.
The cable ends can be unbolted if they're worn and the socket doesn't stay on the "ball"... so you could buy cables from a breaker (or new cables) and just remove the ends and fit them to yours.. It's a lot easier than replacing the entire cable if the rest of it is good.
Ralf S.