Hi Graham,
You can download a copy of the current class rules from the ISAF website by following the instructions here
www.ukmirrorsailing.com/index.php?option...le&id=585&Itemid=330
Going through your questions:
1) Yes, you can now have a vertical leading edge on the rudder blade (but it must not go forward of vertical), see Class rule 3.9
2) Yes, you can have drains into the aft tank, see Class rule 1.5.8.
3) Yes, I think these came in during the early 1990's. Bell/Widebeam used to supply a plywood bulkhead with a big hole in the middle (to save weight), cutouts for the beams and limber holes, see class rule 1.6.1.13. I'm sure Duffin Marine will supply you one (and see below).
At the moment you can't change the daggerboard side panels so they extend aft at the top to form a knee to support a centre mainsheet block platform BUT, there is a substantial update to the wooden boat specification which progressing though the rule change process. At the moment the National Class Associations are voting on it and it looks likely to go through. It still needs to get through the ISAF Class Rules sub-committee, so I can't promise anything. If it passes in its current form it would allow over length daggerboard knees, over length daggerboard case top battens and daggerboard side panels which extend aft at the top to form a knee with the intention of creating a platform for a centre mainsheet block.
It also changes the rules so if you have a kit which omits an optional part, or an option on a mandatory part, you can buy a part from a kit manufacturer and add it in, or make one yourself.
By the way, we have some more up date building instructions than the ones that came with your kit (still not perfect). They are available on the website and were kindly supplied by Mirror Sailing Developments in Canada, more details and a link here
www.ukmirrorsailing.com/index.php?option...ding-a-mirror-dinghy