The 16x12 may be able to be re-pitched to 16x14 if the smaller one is not loading up the engine. Or if a 'bigger' prop is required there may be an option of a 5 bladed 16 inch prop however that would also require the resources of a prop specialist.
I think that Beta recommend an 18 x 12 propeller for the 43 hp, I would imagine that if your present prop is OK in your boat that the Beta 35 would do the job. I would get an opinion from Beta but anything to do with propellers and hp is full of ifs and buts and mathematics. Alternatively the Beta 43 would maybe ok with a coarser prop, 16 x ? I have a Beta 43 in a 10m x 3.4 x 11 tons with an 18 x 12 and it gets along nicely at 1500 rpm.
I am considering replacing a Perkins P4 (1953) nom. 43hp, with a Beta 35 or 43, on our sailing skutje tjalke Alberdina. It has a half tunnel which only allows a 16" prop - at the moment it is 16x12. I wondered if the prop would limit me anyway to getting the full 43hp, so favoured the smaller engine. There is a 38, but this seems to be just a higher revving version of the 35. It is an easily driven hull (18 tonnes 14.7m x 3.4), and I very rarely use more than half throttle. Any helpful observation?