Gauge upgrade question

[CG]

So had this built for me years ago by a board member, sorry name is long forgotten. It's got some shelf wear from being shuffled around but still really nice. What would I need to add a tach to it? I think I've seen someone reproduces them, do they make harnesses too? Will I need new circuit? And lastly my lettering/numbers lines are green, is this going to mess up the matching?

Sweet, pics are upside down. Posted pics from my phone earlier with no probs, oh well