Master Cylinder - Changing on early 911
The master cylinder project is a simple one. On all early 911s, the MC is mounted on the outside body outside the pedal cluster, up in the cavity where the steering rack lives. You may still have a flat sheet metal cover over this area which must be removed. The MC is mounted onto two studs on the back of the pedal cluster, through the ears of the MC, held on by M8 nuts (13mm wrench). Nylocks are a good idea upon replacement.
You will want to convert to the later style 19mm dual-circuit MC. I believe this will require some minor modification of your brake lines. Can't remember whether they used a single outlet or two outlets in those old one-circuit 17mm MCs. You will also get more clamping force from the greater volume of the new MC; this is so common an upgrade with 914s that it is almost standard operation procedure. In any case, a BRAND NEW 19mm should cost less than $100; try Vertex (ads in P-mags), I think they have them for about $80.
Even with some anticipated fiddling with the brake lines, the entire project should take less than 1/2 day. Remember to bleed the ENTIRE system, using new fluid, when you are done.