The stock MC bore is 14mm, and that's a bit too large for best "feel" and modulation. You'll want to find something in the 11 to 13mm range. I have an 11mm on my '78 and a 1/2" (12.7mm) on my '83. The 11mm gives more "feel" and modulation than the 1/2", but at the expense of more lever travel. 11mm, 1/2", and 13mm are common sizes but I don't think I've ever seen a 12mm. That's too bad because it just might be the ideal size for us, giving better "feel" than a 1/2" but not as much lever travel as an 11mm.
Yes, you want to change out the brake lines. Most of us eliminate the two piece line and it's splitter and go with one long line. For lower bars, a 90cm line length is perfect. You will want a small angle bend in at least one of the banjo fittings (usually speced at 25° to 28°) and you place that one on the caliper for bleed nipple clearance. A straight fitting works fine at the top. The banjo size is 10mm .....
A rubber coated clip from the hardware store can be mounted to the lower triple tree where the splitter used to be for a guide .....
Yamaha used a reverse thread mirror on this side. That's one of the reasons I seek out nice, used Yamaha master cylinders. They have that along with accepting the stock lever and brake light switch. This 1/2" MC on my '83 came from a Yamaha Vision (liquid cooled 550 V-twin) .....
This 11mm on my '78 came from an XT/TT225 dirt bike .....