You can get new interior lights from Thailand (ebay), I bought one, fit straight in, looks original and all up cost less than about $30 from memory. No brittle plastic to worry about either.
Friendly enough guy, I recently received some parts from this seller.
Edit: Should add he'll send it surface (cheap) mail if you ask but takes ages from Thialand, their camels don't swim so well. This option expect 8+ weeks.
Cheapest would be to adapt something from a similar sized car. I haven't come accross any great ideas yet. Could try get a generic after market evapourator unit and air con stuff a sunny or something.
Have a look in the wiki, might be tough finding those parts though.
Could be the clutch on the starter gear. (The bit that lets the motor speed up after it's started and the starter is still enguaged.) Check this by anchoring the starter, turning it on and sticking some wood (carefully) into the starter gear. It'll stop if that's what it is. Had it happen on my old HQ. Back then it cost around $20 for the part to fix it (Holden's are much cheaper to fix though).
I'm pretty sure the key just powers the solenoid so if that were the problem it just would't do anything. Usually the solenoid fires, enguages the gear and at the end of it's throw connects the starter motor to power.
He's from Country SA north of Adelaide isn't he? I read a bit on this a while ago. My guess is there's still some issues with emissions/ oil consumption as with any motor that has rings slide past a port/ valve. Brilliant idea none the less. There was a news article or two on it somewhere.
Nice work on the dyno! I have a back ground in control if you get stuck at any point. I know a bit of classical control and state space control and am pretty handy with Matlab and Simulink. I don't think an eddy current retarder is linear but these things can be modelled and tamed.
Nice one! A problem I hadn't gotten around to thinking about yet, I have a switch mounted in the cabin like Levey. Got the same gearbox as you so I'm very interested in a how to/ parts list! A photo of where the switch goes and part number/ description of the switch would be enough.