You do not say what scale you are planning to work in, Mr Hooper's Manx was 1/6th…
As an interim whilst you mould your own, or indeed as an alternative, there have been a number of plastic motorcycle kits in 1/6th, 1/8th, 1/9th from which you may be able to source the tyres – or approach the manufacturers for 'spares'. Of course many of the kits are of Harley Davidson 'cruisers' on 16" rims which are going to be unsuitable, but there is a mix of other bikes and wheel/tire sizes out there….for instance the Harley WLA (18" rims might suit).
Just a thought…
Regards,
Phil
Edited By Weary on 15/05/2014 13:58:09