When I start a restoration job, the first thing I start with is the magneto. You can rest assured that something is wrong with it, especially if it is a Lucas Magdyno. The original armature was insulated with shellac, which after a few seasons will crack and become hydroscopic. The result is shorted secondary windings and a weak spark. The factory seldom used more than 8000 secondary windings, but there is room for ca. 12000, which will give a much more healthy spark. If the armature is insulated the modern way with oven-baked epoxy, there will be no more armature problems. Next problem area is the condenser. Until the late Thirties, most magnetos were equipped with Mica-insulated condensers, which could last a lifetime. The second world war disrupted supplies of mica from Rhodesia and India, prompting manufacturers to come up with a replacement, a condenser with alternating layers of metal foil and waxed paper. The wax degrades with time and exposure to moisture, and you have a faulty condenser. A correct type of condenser for a magneto must be able to withstand heat, cold, oil and humidity, but the main thing is that it must be able to work with high frequency voltage peaks. Very few modern condensers will survive for a long time in this environment – many Lucas magnetos overhauled in recent years have had a short condenser life – 1-2 years has been normal. Charles M. Falco, an experimental physicist and member of the Antique Motorcycle Club in the U.S., has dug into the condenser problem, and tested a large number of condensers available in the market. His results have been published in the Antique Motorcycle Vol. 50 nos 3 and 4 (2011). To make a long story short, he finally recommends using pairs of Panasonic 0.082 microfarad polypropylene-film capacitors. When soldered together in parallel (by soldering the two leads of the capacitors directly to each other), they produce a 0.16 microfarad condenser that will fit into the available space in the end caps of Lucas and BT-H single and twin-rotating armature magnetos. These condensers are available from Digi-Key Corporation (https://www.digikey.com) under part no. ECQ-P4823JU at reasonable prices (ca. 50 p or 74 cents apiece).
You will often find that the armature is slightly off-center in the magneto case. Sloppy work at the Lucas factory. Likewise, the slip-ring is often not ground correctly (this was regarded as apprentice work at the factory – and quality control was not very good). Dave Lindsley will do a good job recentering the armature and will also be able to regrind the slip-ring or supply a new one – preferrably with the original felt lubricating pads. (At least that was what he did on the magneto of my 1936 Squirrel). I have also used Sean Hawker (https://www.hawkerelectrical.co.uk) in Bristol. He has overhauled 4 magdynos for me – Lucas and Bosch. Both gentlemen will naturally also re-magnetize the magneto for you.
I have great respect for the BTH electronic magneto, and have one on my Vincent Black Shadow, where it functions very well, but if you want to keep your Scott original, I would try a total overhaul of the Lucas Magdyno. The BTH is expensive, and an overhaul of the original, with new, improved secondary windings and condenser, will give a very satisfactory result without ruining you financially.