if you're not changing the pump, the engine swop will not make a difference in the way the machine operates,other than the point at which the engine stalls under load. RPM is RPM as the pump sees it,the only gain you will make is less fuel burned per hour with a smaller engine. I'd stay with the 13hp and look into relief valve settings by installing a couple pressure gauges in the various parts of the system so you can see whats going on..after some seat time in getting to know the machine you can sense when you have a hyd.circuit loaded up and when you should back off the valve or the relief valve cut in and dump pressure back to the tank.We have a few here that have built machines(myself included) and can offer advice,be sure to post photos of what you are talking about when you do..some machines out there have had pumps and valves replaced with other than design parts which adds complication to diagnosing whats wrong if a machine doesnt operate right..maybe we can help you.Welcome abourd..