Assuming it is a standard you have to keep the 240 in good tune. Basically because of a simple fact. Like early flat four volkswagons you need everything it has or is capable of producing. If your top speed is as indicated and it is a standard it needs some work. Timing chain check,timing check, injector check, make sure all filters including the tank screen filter are flowing well. Good tire inflation and not semi seized brake calipers etc.etc. Critical examination of the throttle linkage as already mentioned is very important. When you are on the pedal the stop on the final pump linkage must be reached.
Then the comfort items as they seem to all scream at different volumes at high speed. You need or want good engine and transmission mounts, good hood pad. Good sterio to help cover the noise. All things in good shape and you will think you are going ten to fifteen miles slower in my opinion.
If using the car a lot then the rear end from a non turbo is not really a waste either. Also probably the easiest rear end to get hold of as well. Seven percent change is a lot for these bumper cars. I own two of them. The automatic is another animal and I have no suggestions for it other than a possible conversion to a four speed. You just cannot spare the few horses lost in the cement mixer. Also keep your trunk load light and glove compartment at low capacity. My red one at high speed earnt it's name. Bedlam.