Uses standard MSD chips - right?
If so, just use a 1.5k ohm pot. Go to radio shack or wherever and get a 1500 ohm potentiometer. Center tap to either one of the other tabs and you are good to go.
I would need to look better at my data, but I believe 1800 rpm would be around 650 ohms and 2000 rpm would be around 860 ohms.
You can also use regular resistors.
For my leave point, I use a modified potentiometer and use a 10 turn pot with a locking dial. I just look up the rpm I want on my spread sheet and set the dial and I'm pretty close.