factory tachs?

[jonzcustomshop]

I am hoping one day to find, and put factory tachs in both my 61, and 65.
I have done a little research, and found out a couple things:

1)they are super expensive, unless you get lucky find.

2)I had seen something about them not being compatible with hei, or more modern systems, I think I also heard about a seperate tach drive/sending unit?

so my questions are, what is needed to make an original one work, and, is there a current tach on the market that is the right size, and has a similar sweep like the factory ones? (I have a friend that could replicate the factory style numbers on a sticker, that I could put on a modern tach.)

here are some pics for reference: