Welcome aboard Vixen. There's always the possibility of transplanting a Triumph straight six into a Herald chassis. Phil J's BRG example started its life as a Herald Courier van, but now runs a Triumph 2500 straight six -
Good luck finding the right donor, depending on your budget this might give you a good start -
http://www.ebay.co.uk/itm/AWESOME-Tr...item337d6ac273
It even has a 3 letter 3 number registration plate which alone would cost over £700 to have transferred to a younger vehicle.