What tyres are we talking. I have various well treaded old 4.40 by 10 or even a few 4.80 by 10 but most of these will not be suitable for road use simply because they are old.
I believe there are still small 10 inch tyres available new.
More Michelin were found in Italy and bought over, but your talking money as they are a good tyre. Ian Frankland at Taffspeed.
There was a Kings or Bridgestone 4.00 by 10 available, which I think the Trojan guys were using. It is quite a nice bulky tyre belying its narrower section. Nick Haddon will know all about that.
Uwe Staufenberg stocks tyres for Goggomobil in Germany, which will fit.
For MIni tyres there are two cheap sources. One mail order in Workington and the other on eBay in Spilsby, Lincs. Both sell Falken at around £28 - £30 delivered. I fear the 5.20 section is to big for a Frisky though. Also being radial the handling will be different ie they grip better so it is easier to flip the car as no warning skid, squeal or juddering. Radials need more power to drive too as they deform giving a softer ride. They work out well on a Goggo but not without some modification to prevent restricting the lock of the steering and rear clearance but that is a four wheeler. Nonetheless you can reach critical swing arm inebriation attack before you know it, as again, no real warning from the tyres.