Recently I got the 18hp installed in my V2, it was a direct bolt in using all the old parts needed to make it appear stock. Even with my severely worn drive clutch there was a noticeable difference in performance, the machine had more pull to it.
Would I have done this if I didn't have one or need to pull the engine to change the stator?, probably not, but I do like the difference.
I have had the engine on a shelf for 12 years and did nothing to it but change the oilso adjusting the valves and a tune up may help a little more, but I'm happy.
I will keep the original engine and probably will go back in at some time as the 16 did its job for 22 years and 1140 hours with no issues until I pinched the stator wires replacing the starter drive gear. I pulled some serious hills with 4 people in the machine over the years, you just need to learn the machine to accept how it does it.

If you ever go this route, I would recommend going with the 18 for ease in installation, the 20 would be near as easy with the throttle linkage being the biggest possible variable. Be advised though, the newer engines all come through with sealed carbs for emissions and the 16 carb is smaller than the 18 carb.

Good luck