Even though there's what looks like a seam between the lower and upper dash panels, I believe that they must be removed as a single unit.
There are many after market tachometers available for gasoline engines that use a spark sensor of one sort or another. Here's one:
Tiny Tach
There are many after market tachometers available for gasoline engines that use a spark sensor of one sort or another. Here's one:
Tiny Tach