Exploring the possibility of fitting rotors and calipers from post 1991, which are bigger, to my 1990 turbo. Will they bolt straight on or do I have to change struts as well., any advice would be great, billr

straight bolt on mate, all you need are the bigger brake calipers, as well as the rotors and pads to suit.

the backing dust shield just needs to be cut/bent to clear the bigger rotors, or you can remove the hubs to swap them out too. I just bent them back with pliers. no biggie.

Good luck

Although the early 7/8" master cylinder will work along with the early booster, for best results you would also need the later model 15/16" master and a later model more powerful booster. 
Post 11/91 boosters are different to the early ones but the best one to get is the post 11/93 one. 
Maybe read all of this:
 https://www.mr2australia.com/mr2play/tm.aspx?m=80895