Is the mounting you need a screw or rivet on thing or is it originally welded to the rear subframe ?
Normally they are just a piece of flat steel, sometime's bent with a right angle that's welded to the frame
If that's what you are after, then you could buy a piece of steel or alloy strap, drill the mounting hole, bend it to suit if needed, and screw it to the frame, or even pop rivet it to the frame
Then use a nut and bolt to secure your plastic side number plate to it.
If you want it to look original then you could have someone mig weld a nut to the back of the bracket, then weld the steel bracket onto your frame then paint it after it's done
Just re read this question, do you mean the R/H tank shroud mounting ?
Kawasaki part number 11055-1636
OEM Kawasaki Dirt Bike Parts Up to 2 % Off | MotoSport
11055 in the pic below, $20 AU new