Look and see if the oil pumps are still on the engines first, they are located under the carbs. with a short cable going to them off the primary throttle linkage.. If its on premix I would leave it alone, I converted mine several years ago and have been running Klotz R50 40:1 with no problems at all, just a sweet smelling exhaust.. It costs a little more to premix but its cheaper than new engines..
You can try and adjust the throttle cables and idle air adjusters.. I found the best setting for my idle air screws is 1 turn out from lightly seated then adjust idle stop to desired RPM.. Another thing I have noticed on the 96 speedster is that the tacometers are not always correct, you can listen to the engines at say 3000 rpm and tell if they are in sync.. I have been driving my boat and the tacometer readings vary from 1000 to 1500 rpms between engines without touching the throttle then they go back to an equal reading, just a glitch in the tacometers..