So after learning lots of lessons about wheels, hubs and spokes, I finally found the right parts to lace my back wheel! Here is the end result, the first rideable version:
I'm not happy with a couple of things with this - firstly the motor is too low and the clearance is not great. This is because I had to use a slightly larger rear wheel which is my second problem.
I am using a 32 hole hub, and it wouldn't lace into the original Dirtsurfer 28 hole rim. I looked all over for various ways to fix this but most people suggest not trying to lace a wrong hub to a wrong rim. I eventually went with a new rim, with 32 holes to match my hub. For anyone else thinking about doing this, there is lots of other options. There is a 36 hole version of the hub I used, or you could used a flip flop hub with screw on disc brake adaptor.
What I didn't realise is that not all 16" rims are the same! The Dirtsurfer original is a 305 rim, and I ordered a 349 rim - the 349 rim is 44mm bigger - so it rubbed against the motor with my new motor mount
The choice of 305 rims is very limited so I need to rethink this area. The 349 rim also suffers from a lack of tyre choice, there isn't any offroad/mtb tyres for it, so I've got some horrible road tyre on it right now...maybe good for power slides, but not good for grip on grass
Anyway, still exciting because this version is rolling and rideable, and so far - all good. Just had a very short and quick blast in my back garden and so far it seems pretty good. Cogging at low speed was always expected, and it certainly does that. It also requires a very smooth trigger finger whilst kicking which is proving tricky, but I'm sure practice will iron everything out.
I should probably wait until I can get the motor tucked up a bit more before I ride it properly, so updates coming after that is sorted.