Hi there, I stumbled across your question (very apt for my style of walking) as I am the proud owner of 2 PDQ trikes a MK1 and a MK2 "Extra" with the newer "upgraded" control box. My first was purchased through the ever present eBay and ran for just long enough for me to fall in love with being able to go out and about again then the circuit board fizzled out and after much searching we (well my excellent local electrical fabricators) found the contact details for Davinci and you can still purchase the control boxes at the knock down price of ......£170 + £30 p&p or £180 +£20 p&p. I found it quite interesting that both quotes although different added up to a nice round £200!
Whilst umming and ahhing about what to do I found a complete set-up of Power Trike Extra, Liberator wheel chair, extra off road tyres, carry/storage case (I now have two of those too as I had just purchased one of.....yep eBay again) a normal lead acid battery and the much better Niad battery which sell for £450 to buy from Davinci and it roughly doubles the distance, double battery carrier etc etc etc in perfect working order sold as seen.....got it by courier didn't work and after the first 2 days 1200 Euro's down the barking drain! I decided to bite the bullet and but a new controller from Davinci for the newer trike....they no longer hold them as a stock item, can't get them and don't know the company that makes them......but they will take the trike in and retro fit the older system onto the newer trike...but that would involve a new circuit board, wiring loom, controllers etc etc etc at considerably more than the cost of just the faulty circuit board....so began my search for alternatives....
Right down to business, it is possible to find replacement controller boxes on eBay for the size of motor the trike has for >£30 I picked one up for £16 + £2 or £3 p&p but have not got the cagoules to start chopping the wires to fit it!
But, I have recently found a company who make good quality controllers and can provide a controller pre-wired to a new twist throttle speed controller as the origional ones can (so I have been told, but cannot confirm) be part of the problem with the circuit boards over heating!
The Company e-mail address is http://www.4qd.co.uk and the controller they think will do the job from my description of 250w motor (it may even just be a 150 motor) and 36v batteries they replied to my e-mail coming back with what I had thought may be the best one and I had also seen on a forum somewhere but cannot remember where now
All info is in their site. and they suspected the following application:
http://www.4qd.co.uk/prod/ptr.html The Porter controller
http://www.4qd.co.uk/accs/pots.html#mag The twist-grip throttle
The beauty of this system is that it has only 6 wires for you, me, us to mess around with 2 for the battery 1pos & 1neg same for the motor and again for the throttle assy 2 more wires. I don't know if you have had a look inside the trike control box yet, but there are a lot more wires than that as there is ignition and battery power level indicator but I'm sure with a bit of doctoring I could pass the positive wire through the ignition key and also wire in either the trike battery indicator or another one from....the provider of all things

ever needed..... eBay!!!
The above set up is around £100 cheaper than the Davinci weak control box, its not just us they are lying in the back of garages all over the country because of the same problem....yet the company still insists in fitting the same crappy circuit boards! The Porter Controller is £47.25 and the Magura Throttle + 1m CONI lead £43.58 and a matching grip for the opposite side of the handle bars comes in at £2.10 totaling at £ 92.93 but of course it could be worth a punt to try keeping the existing throttle assembly. After all if it does go wrong we know where to get a one that does match the circuit board/controller and that will put it at under £50 either way its cheaper than Davinci, a far simpler system and backed up by a company that specialise in these type of things. They started out producing golf buggy controllers, which have to be sturdy and take some heavy, abusive use and last the lifetime of the buggy. The difference between Davinci and 4qd is one had a captive audience and the other had to make something that works and compete with the competition if they produce poor quality they will soon be out of business!
I hope my rambling yarn makes some sense and points you in at least "A" direction even if its not the one I am investigating (and will in truth be purchasing in the next few days I am just deciding whether to go with the old throttle or stump up the £44 for the new and guaranteed one.
Charlie