Hi Andy, snap! - I have a '34LH. unfortunately, dad was only part way through the resto and much of it is in boxes.
However, I can answer some of your questions. The head would have been a single plug. I don't know where a second plug would go. Any chance of a photo? The clips mostly broke over time. The covers - and maybe cups under the springs (not sure about that) are available, but cost an arm and a leg.
There's about 3 different oiling strategies for the guides over the '30's. I think '34 has a breather from the rear of the crankcase to the inlet valve. It goes into the side of the head.
My head has serious porting and big valves. I had it club dated and apparently it was factory issued with a High comp piston. In Aus, a number of LH's were raced. I'm not sure why. This might be the same with yours?
I don't think it should be a plug in the centre of the head. About the only sensor used in '34 would be a thermometer...
You'd have to replace most of the motor to change capacity. The rod is unique to the model, possibly flywheels, certainly head and barrel.
I understand they are far quicker than you would expect.
Also, they had 20" rims, front and back and a pressed steel (not cast) front drum. I'm not sure the frame is the same as the larger bikes of '34. I'd need to measure it.
Mine has a pressed steel primary and the smaller brake pedal - late 20's style.
I'd also be interested in a photo of your front guard and stays.
I don't think your model is particularly common...
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.