I don't think 30mm birch ply would work for the servo blade, though you might try it for the servo carrier if you can find dinghy rudder fittings to suit. You really need a natural timber for the blade, not necessarily hardwood, a dense coniferous timber would do. The servo blades for the original Hasler gears were made from spruce, and I always thought that was too light, as they tended to float up to one side, whereas self-centring is desirable. Once, I had to sort a Hasler gear where the spruce blade had warped, and it was difficult for the owner to see why it was steering badly.
If you can find a scrap of 76mm or 80mm aluminium tube, you could use that in place of the 80mm GRP tube (offcuts from my yard) that I used. The 10mm and 6mm carbon tube for the vane can be found in model aircraft stores.