I've had built-in tachs on a number of boats...
Never had a Tiny Tach . Some people recommend them. I think that they can be affixed to any motor by winding a wire around the spark plug wires.
https://www.tinytach.com/tinytach/index.php
Candidly, I never look at my tach anymore. I can pretty much tell if things are going well by the whine of the engine and by observing the speeds etc.
If I were analyzing an engine ( which I wouldn't really know how to do ) then, I guess. I'd want a tach
re your wire issues.....Here is a wire gauge/length calculator Hope it helps
regards, rich
https://boatstuff.awardspace.com/awgcalc.html
I added this ^^^^^^(better boat qauge calculator)