OK, I spent one night recently doing a soft restoration on the yellow Sharknose Ferrari and red Lotus and fitted the Carrera tyres as mentioned above... The tyres are way too hard (even new out of the packet) for such light (non magnet) cars, so this morning whilst having breakfast I tested two sizes of Ortmann Tyres that I thought might fit. Both gave excellent results on the track from being undrivable to being right on the pace with all my other faster 1/43 (but standard) non magnet cars.
All I had to do to get them to perform was go from my usual non magnet 3 volts regulated (for Artins) to 4.5... As they seem to have very soft motors, which was typical of the era and actually my preference
PMTR4572 which is a tight fit period slick from the Revell Monogram Porsche 550
PMTR4582 a slightly loose fit Ninco Classic size
The cars are noticeably smoother and quieter on the Ninco fitment, which I think is due to the flexible tread acting as a dampener.
I'll grab a set of treaded PMTR4573's (Revell Monogram Mustangs) also, as I think they might fit too... I'll confirm ASAP.