I thought it was only the 350CC WNG model that had the longer front fork? The forks needed to be longer as the frame had a higher than standard rear section so the forks and frame are matched.
So ....David it may help if you let us know the frame number so we can see exactly which frame you have.
There has been an original set of WNG forks on Ebay a couple of times from Greece. I don't think they sold (last sale ended yesterday) see the listing here:https://www.ebay.co.uk/itm/Ariel-350-NG ... 7675.l2557
And another sale in UK from last year that didn't sell so seller may still have them:https://www.ebay.ie/itm/391967888750?Vi ... 1967888750
Jake Robbins in the UK is reputable girder fork repair specialist, it may be worth giving him a bell:https://www.jake-robbins-vintage-engineering.co.uk/