I have no experience with the 50cc scoots, so can't help with cvt tuning......but as far as a tachometer goes.....I put one on my scoot over 4 years ago....still works fine. It is like the one in this link, only now they have replacable batteries...mine doesn't. Easy installation, just wrap the lead around the spak plug wire 3 or 4 turns, attach tach with 2 sided tape, or zip ties, and off you go.
http://www.ebay.com/itm/Black-12VLCD...NXtSZJ&vxp=mtr