Pete,
If you want to build an A class there were plans available for the Goodall Mk2 and updated by hand to bring it line with the Mk3.
(Not sure if you can still buy these.)
I think Mk2 is 1984 vintage but the hand written updates should bring it up to late 80s vintage.
The hulls are supposed to come out at 22kg.
If you are going to race it I think the competative weight of todays A classes hulls is around 17-18kg.

If your not going to race there are more robust designs out there than an A class. An A class is only 3mm ply while boats like a Cobra, Mosquito,Taipan and Blade are all 4mm and while it does not sound much difference the 4mm is a lot stiffer and will take impact a lot better.
I've built boats in both.

I had a friend that built a set of Marmian A class hulls a bit over 10 years back and he got it down to todays min weight but he was a master of light weight construction. The average person just isn't in the hunt.
I don't think the Marmian plans are available anymore.

Now if there was a Flyer type hull design out of ply you might make it because the Flyer has less surface area which is less ply and less internals etc.which would most likely get you into the ballpark. The hull shape has also proven to be much faster that is why the guys at the top have them.

(I was working on a design but had to shelve it to concentrate on the Blade F16.)

If you want to build an A my suggestion would be to contact Greg and see if he will still sell the MK2/3 plans. The plans are quite cheap but they are really only are for the hulls the rest is up to you.

Good luck.

Regards,
Phill