Hi ya David
May I ask you to err on the side of caution; as when increasing the size of models from Plans and altering the builds; you can run into all sorts of problems. Maybe not so many problems for an experienced builder, but for a person who is relatively new to the hobby - personally I would advise you to build this model as it (straight from the Plans) and this would enable you to see how the model goes together, how you actually fit up the power drive chain and so forth. Whereas what you are asking to do - which is quite possible to do - you can start to run into problems.
When you start to diagonally plank like you want to do and also increase the size of the model - you will have to begin to think about putting extra support in - e.g. stringers on the sides and on the bottom - these are strips of wood to support the diagonal planking in the middle of the planks.
Basically this design for this model doesn't lend itself too readily for doing this. So that is why my suggestion is to stick to the plan size for the model and minimise the problems that you are going to create for yourself.
If you are really into this type of style of model - the Marine Modelling International Magazine (last month) did a free plan for an RAF Rescue Launch which I believe Stavro is building. This is an actual twin motor model and it looks a very interesting build but built of a similar style as Glyn Guest. Also a while back the same magazine did a free plan for a Vosper MTB semi-scale and this one is the same as the Airfix MTB is.
When you have built your skills up; you then possibly may go on to building from plans such as Vic Smeed's plans/John Pritchard's plans. Have a look at MyHobbiesStore at the style of plans and also have a look at the MasterClass build and you will see an Air Sea Rescue Launch which I built and diagonally planked. Also if you do a search you will come across the RTTL which I built. This is also diagonally planked.
Hope I haven't put you off too much and dampened your spirits but good luck with what you choose.
Last but not least, going back to the Glyn Guest MTB; you can really add a lot of detail to this model and make it look something really nice. Do a web earch on the MTB