Henry
There are no plans available for the H31 at 1/11 scale (34") but the MyHobbyStore X-List plans service has a 1/12 scale (31") plan MM1061 by 'Model Boats Staff' (aka Alec Gee and Vic Smeed).
If you increase the recent 1/16 scale Model Boats magazine plan by 25% you'll get a model which is around 30" long, or 5/64 scale! At that size I wouldn't increase any of the wood sizes or types, except maybe add an extra 2mm thickness to the keel and fit an 8mm dia prop tube. At 1/12 - which is almost identical in size to 5/64 - you could afford to use liteply instead of marine ply, which was necessary because of the engine used in Vic's model (a single-cylinder ED Viking diesel). Well, that plus the fact that liteply hadn't been invented at the time!
If you go up to 36" then marine ply does have more attractions, certainly for the frames and keel parts.
Harry's choice of motor seems too powerful to me, certainly for models of around 30" long. My second model 1/16 H31 (with the tan seats and swimming ladder) has a Leopard 2826/17T 1050kv motor on 3S LiPo and there's a pronounced torque 'roll' even at 3/4 throttle. I've also had to restrict the throttle to 80% as the model at 100% speed scares the hell out of me! Next experiment is to try a 2S LiPo and then maybe a 750kv motor on 3S.
It's all good fun!
Dave M