1. To split a rudder do you simply take a cutting saw and run around the edges?
You can slit it with a cutoff wheel in a small grinder or grind it down until the seam opens.
Stem part of rudder has obvious seam but leading edge is not so distinct and it seems more difficult to keep a nice straight cut. How do you join back together?
You join it back together with fiberglass "tape" - you can buy glass fabric in rolls of various widths.
If I elect to simply run a length fibreglass tape along the aft stem seam and the forward leading edge seam what do I do to prepare - there is a layer of Interprotect and then antifouling - do I sand these down to fibreglass? If I do that I go through gel coat? And if I do that do I have to reapply gelcoat or just Interprotect again? Just not sure how to prepare surface ans what ramifications are. Have searched internet and Sailnet but no description found.
You grind or sand back to clean glass - no paint, coatings or gelcoat. This is critical since you are going to be creating a secondary bond which inherently has less strength that a primary, chemical bond. Use epoxy resins - they "grip" much better than other resins in this situation and they are far more waterproof. You don't need to reapply gelcoat afterwards.
In fact, when working on old boats, I'd recommend forgetting that anything other than epoxy even exists.
I second Smurph's comments - if I was you I'd so some reading up on laminating, tools & materials and make some practice laminations before cutting your rudder apart. Glass work is pretty simple but there are some important things you need to know beforehand. There are lots of books with this info in them.
Check out Lackeysailing.com - he shows all aspects of boat restoration by documenting the boats he rebuilds - it's a great site.