It's not difficult to install an original mechanical tach, just expensive.
You need four components:
The tach
A tach drive
A cable
A bracket to mount the tach.
There is a tach on ebay right now, item # 391136027879, and another # 181802898299. I am not endorsing buying either one nor am I endorsing either seller, I just mention it so you get a feel for pricing and what the correct tach looks like. You will see some without the redline, but the Ducati's all had the red line just above 8000 RPM.
The drive is a matter of changing the cover on the top of the bevel gear tower for one with a mechanical drive that is fed off the bevel gears. See ebay item 281489548326.
You can fabricate a bracket at that point or I can point you to reproduction brackets. The original bracket would have clamped at the base to the headlight mounting stanchion on the driver's right side and to the top of the right fork cap. However, the geometry of the whole thing may be dependent on using the short headlight stanchions made for clip-ons. I am not sure of the geometry still works if you are using handlebars and tall headlight stanchions.
I have seem race bikes fitted Scitsu electronic tachs:
http://www.scitsu.co.uk/docs/main.htmI don't think they are inexpensive either.
Another thing to consider with an electronic tach is what kind of power it will need; 6-volt or 12-volt. And one step further is whether it needs AC voltage or DC voltage. Some of our old Ducatis were completely AC 6-volt bikes, and if the Scitsu or other electronic tachs require DC voltage you could inadvertently destroy the electronics in the tach.
Jim