I had a +4 on an L plate from new. The performance of the car was transformed by a Librands exhaust, replacing the catalyst with a straight through pipe and by work conducted by Colin Musgrove Racing. CMR rerouted the air intake to remove the obstructive kink shown in raf 1's picture, fitted a K&N filter and replaced the convoluted exhaust manifold with a later variant from Morgan. I will never forget the demo run that Colin gave after the work - it was fast.
Sadly no Morgan at present but a Triking is on its way.