There are two types of GPZ900R mirrors. A7 or later and A6 or prior. Kawasaki model name started at A1 and increased by every year.
Probably ebay mirror is older type. The older type does not fit according to Japanese CBR750 website.
There are many after market mirrors. But many motorcycles bolts interval is 35 - 55mm and it does not fit.
According to my memory, SC21, RC24 and RC36 have smaller interval than RC27.
Under the mirror base cowl, there is a steel frame which has mounting bolt hole. So if you want to make another hole, you may need to use drill for making a hole. I don't like this method.
According to my memory, availability of new genuine RC27 mirrors is quite bad.
About couple of years ago, I heard Honda did not have the parts. One of reason for poor availability is, considerable number of mirrors were purchased by passenger car owners in Japan.
By the way, this mirror thread should go to tech section. I think.