I think my first move would be to get the radiator turned the right way round. If it doesn't fit the right way round then one that does would be a good move. They are designed to work with the elements vertically positioned.
No matter which RH model (or any kit car) you have or prefer, it all depends on how well it's been put together. I have seen good and bad examples of most of the models over the years. Each individual car needs to be judged on its own merits.
My 3a was first registered in 1998 and just missed the introduction of IVA. It also has the advantage of a Q plate which means the emissions are visual only. Its not a problem as all in good order with no visible smoke. It is rather nice to know that no one's going to stick a sniffer tube up your exhaust pipe. It usually takes my guy about 15mins to test with me in the car as he is a little on the plump side.
I have a 3a with inboard coils. The standard/lit supplied springs are much to strong. I thing they are around 300lbs. I replaced than for 125lbs ones and the difference in ride comfort etc is amazing.
I had twin Webber 45s on my 2L pinto. They were quite old and very difficult to get and keep balanced. I swopped them for a set of Yamaha R1 carbs and have never regretted doing so. Obviously you should not have problems with new carbs though.
I have a 2ltr pinto on Yamaha R1 carbs. The vacuum dizzy is connected to the carbs via tube only. 1 to each cylinder and 1 for brake servo. All "T"d into 1 tube to the dizzy. No chamber at all. Pressure regulator set at 1.5psi. everything works fine throughout the rev range. I would think it's a carb problem on the lines of Peter Bells suggestions.
Re the above comment about stainless steel and magnets. Some grades of stainless steel are magnetic. Ferric stainless, as per the body used for the stainless mono robin hoods for example. Pure stainless is non magnetic. Never tested the tank on my 111A to be honest. You can weld stainless but you need someone who knows what they are doing.
I have Yamaha R1 carbs on my 2.1 pinto. The distributor is a vacuum type and the set up works fine. As long as the engine is getting fuel delivered to it the dizzy will tell it when to fire. If the engine is running okay with the vacuum dizzy set up, it should be fine with the bike carbs. All you are doing is changing the fuel supply. You will need a pressure regulator in the fuel line as bike carbs only need a low pressure.
Dave.
I fitted Yamaha R1 carbs on my 2.1 pinto. You would obviously need the correct inlet manifold, but well worth the effort. They are quite a simple carb and once set up are relatively problem free (famous last words).
If the seals have gone in either the master or brake cylinders you should see evidence in the form of leaking brake fluid. No resistance in the brake pedal travel and an audible hiss points to the servo as stated by "spare part" above.