Manual to electric tach.

Hooking up the new electronic tach will be no problem. I was wondering what I can use to block off the port on the head where the manual tach cable comes out so I can run without leaks. Can I just cut the cable off and use it as a make-shift plug??

Sure you could do that, but it would probably look funky like that IMO.
An easier and cleaner looking way would be to remove all of it, then use JB-Weld to fill the hole flush.
There's not enough oil pressure in the engine to cause it to burst or leak, and it will stand up to the heat just fine.
This is what I eventually plan on doing when I swap over to an electronic tach on my 650.

cant remember were i seen it, but one of the company that sell kz parts has a insert to block off th tack hole

at Z1 Enterprises they have a block off just like you are talking about. Heres the link: www.z1enterprises.com/detail.aspx?ID=2647