tachometer on 40HP litter

Hello,
I am looking to putting on a tachometer on a 1984 40HP Yamaha tiller steer motor. How hard would it be to install a tach? Also it's a pull start, do I need to attach a wire to the battery? Where would be a good mounting location, could it be mounted on the engine cover? Finally, what would be a good brand to use?
Thank you.

[boats.net]

It would require a battery, a on/off switch to operate tach. The tach requires a 3 3/8 hole so you wouldn't be able to put in cowling.