Yes, probably they are all different ;-)
I got this response from Steve at Conways, June 2020 :
Nikasil coated 90mm cylinders complete ? £600 each 8 weeks
Pistons 90mm (8:1 or 9:1) ? (Make ?, Forged / Cast / Low exp. ?) £190 Cast low exp, either 8:1 or 9:1 2-4 weeks
Conventional cast iron cylinder liners in 90mm bore for use with my own bored cylinder muffs ? £150 each 4 weeks
Will I need special cylinder studs for your 90 mm bore cylinders of either type ? No
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I ended up buying a set of Terry Princes 600cc Top End Kits with 92mm bore, the dimensions for fitting that can be seen here :
https://www.classicmotorbikes.com.au/tpv-top-end-kit-fitting-instructions/
(I have not fitted it yet).