Hello Jok and welcome to the forum.

The Rasmus is a great boat and, as you point out, you really could use either the Beta 38 or the 43. I'll explain the differances to help you with your decision.

First, your old Volvo was not a 75 HP engine. It actually developed 61 HP at 4,500 RPM and 44HP at 3,000 RPM. There are notes in Volvo's about 75 RS which is revolutions per second, not HP.

The Beta 38 is a lighter and smaller engine than the 43 and it provides more than enough power for a Rasmus, even with the 120 amp alternator in use. Being smaller, you will have better access around it for maintenance.
The 43 is a larger engine, closer in size to your old engine and it has more torque at low speed which will help if you fit a refrigeration compressor. We can also fit a 175 amp alternator to the 43. This alternator is too large to be fitted to the 38.

I have run some propeller calculations for you and the results are as follows:

Beta 38 with 2.45:1 transmission - propeller called for is 17"x 12" three blade with 50% BAR

Beta 43 with 2:1 transmission - propeller called for  is 17"x 13" three blade with 50% BAR.

Performance should be much the same with either choice.

Have fun, Stanley