The analog to nmea converters discussed so far allow you to monitor
engine data on a chart plotter. In reality, they are not that useful because you need a second display to monitor
them efficiently. A second display will cost you as much as the analog gauges you already have.
If you want to control your engine, you need something called electronic throttle control, for example check ZF Mathers MicroCommander series. It is quite expensive (about $3,000 for the control module, $500 for each station) and installation
is involved. I am not sure if it makes much sense on a sailboat. It is typically used for powerboats with multiple engines and multiple control stations. Theoretically, it would allow you to control your boat
remotely (i.e. the control stations use components that are nearly identical with RC airplane actuators) if you are into that sort of thing.