Couple I can think of for an original 19" rim...The speedo drive, if standard off the gearbox, will be more accurate, and raising the rear ride height will give an improvement in visual aesthetics, rather than looking to have a softie on the back. Could be the 18" wheel is from a Unit Twin, and chosen to enable rear wheel speedo cable drive and a cheapo speedo head. You did say it was a bitza!
Swarfy.