Peter -that's a really good question, so did some further checking on pistons if an alternative replacement needed to be found...you'll like this, I have an old VW T3 Caravelle Oettinger poptop form 1989, had it a very long time, been round the clock and 200thou the other side -ripped out all the VW transmission and engine -now runs a chipped 2.5 Subaru + ECU, made a cable gearchange, rear discs and I reversed the diff in a WRX Subaru gearbox to run backwards at a higher ratio with Exedy clutch...it's a Autobahn cruiser changing into 5th at just over a ton, top end god only knows, just plenty there and never wanted to go that fast in it to find out, it will keep up with Porsches and leaves similar sized Range Rovers for dust -quick off the start too for a 2 ton brick, but I only drive it around at 50, and 70 on motorways as it's our holiday home and super reliable. Point being, the pistons are good and super strong -and are exactly 99.5mm -100mm with rings -£60 new each and come in a variety of compression ratio's available ...the Nicasil barrel I have no easy answer for if that goes, except pleading cap in hand to Conways/Godden for another to be bored and coated...dread to think, standard £1500 -custom sizes be a fair bit more I'd guess..
Only other way would be to extract and machine a Subaru Barrel from a block -plenty about (Nicasil lined already), thereby creating a liner to be inserted into a over-bored Vincent muff with wider bolt hole diameter...that would work.possibly -doable anyway -but would have a shorter stroke, although there might be enough to get 100mm -they run 79mm, but barrel is longer than that because of different compression options offered across the range using the same basic block, have to check that one...anyway they're already in the form of a liner within a water jacket, and easy to remove and machine, two per side...
A guesstimate be something like -
Bore of 100mm divided by 2 = 50, then squared,( 50 x 50 ) = 2500 x 100mm (approx stroke) = 250000 x 3.14 = 785cc roughly(ish)...
