I don't think it's a specific tach, it just has a tach output so you can use a universal automotive tach... of course it needs to be able to stand up to whatever vibration, moisture, and dirt it gets put through so that really limits the options. Some aftermarket ignitions (like the dynojet PCIII that i have on my 700) have an optional LCD display that plugs in and displays rpm and other statistics depending on what other sensors are installed, but i don't think dyna has one, at least for the off-road models. Seems like the LCD display would be in a highly vulnerable position to be mounted anywhere easily visible without completely taking your eyes off the track anyways.
I doubt you'll use the tach if you just ride off road. My dirt bike has a tach and the only time i even take a glance at it is when i'm stopped or on the pavement. When on the dirt there's too much going through your mind to worry about how fast you or the engine is going.
And about the curve settings... i don't know what to tell you. Deathshadow posted up a couple screenshots of the programming software when he got his a couple years ago, but those are probably long gone by now.