You will encounter a few issues intalling a different make of engine/transmission. These include different mounting location for and aft and also more difficult to overcome is the bed mounting width. There will be different hose and exhaust positions and diameters and the exhaust and or the raw water intake may require replacing all the way to the hull inlet/outlet. There are other smaller changes, but the abovementioned are more difficult and expensive to overcome. Engine size could be a problem, but with current engine choices the sizes are generally smaller than what you are replacing.
It makes things a lot more difficult if the engine choice is limited by your choice or by what and installer wants to supply you. Yanmar, Westerbeke and Volvo-Penta only supply engines with on mounting arragement and it is usually left to you or your installer to overcome the problem by rebuilding your engine beds and/or motor mounts. You might like to give Beta Marine a call, they specialize in re-powers and have a choice of different motor mounts for their Kubota based engines and will make custom mounts for a nominal charge to overcome most problems.
Finally a word about the H-R Rasmus. The 75HP engine fitted to this boat was a choice made by H-R as the boat was designed more as a Motor-Sailer. Doing calculations based on a specs of the Rasmus as a normal 35ft sail boat the engine power requirements are significantly less at around 40HP as I recall, this represents a significant saving if purchasing a new Engine/Transmission.