I would start by identifying the pump you currently have. You should be able to find a flow rating tied to an RPM. Let's say your pump produces 40 lts. per minute at 2,000 rpm. Let's say the pulley on the engine is 6" in diameter and the pulley on the pump is 4" in diameter. As engine pulley is 1.5 times bigger in diameter, it will turn the smaller pump pulley 1.5 times faster. So, if the engine is turning 2,000 rpm, the pump will be turning 3,000 rpm and theoretically pumping 60 lts. per minute. So, if this were the case, I would be looking for a 60 lts per minute pump if I were going to direct drive it. This all assumes you had adequate flow with the original pump.
To recap:
You need to know how much flow at what engine RPM you had.
You need to select a pump that will give you the same flow at that same engine RPM.
You can have 4 different engines that will produce 75KW at different RPMs (1,800, 2,400, 3,000, and 3,400). A direct drive pump will likely only produce optimal water flow for 1 of the 4 because of the RPM. The others, theoretically need larger or smaller pumps.
Can you identify the make, model, etc. of your original pump?
Ted