That Sandvik cutter wheel looks space aged!
Some pics would be good, I wonder if the bolt holes etc line up for a straight swap or what?
BeaverB01 maybe yours is older but it looks like 3600 rpm for you also, probably drops down to 3200rpm working. Get a tach on it and see how it's going. Can you take the guards off and measure the pulley sizes OD and the cutter wheel dia to the tips of the teeth.
Also, any chance of pics?
What sort of air cleaner is on it? I find that switching to Donaldson with centrifugal pre cleaner helped for sure. Here's a pic of one on an RG20 but it only has the hat not a centrifugal
http://www.raycomfg.com/rg25hdsolo.htm
Also have a look at where the Govenor spring is attached, I find moving the spring out to the furthest setting is best, on my Honda engine I actually had to put kinks in it with pliers so it was tighter ... govenor works better then but Honda's govenor is nothing like Kohlers for response.