What does the Owners Manual say about tyre sizes ?
Bear in mind I know zip about Dragonflies (!), but that sounds very much like a (very common) 19 inch WM1 or WM2 size rim.
If you wander down to your local purveyor of tyres, he will have a range of classic 19 inch tyres,
and bikes of your era would generally have had 3.00 or 3.25 or possibly even 3.50 on the front,
so as say 3.25 x 19 and probably on the rear as well.
The front likely was a ribbed type, and the rear a studded type.
If you went with say Avon tyres, these could be like this.
If you want something more exactly like it had originally, he could likely oblige.
Modern compounds are sooo much better though.
Bear in mind too that tyres come with speed ratings, and direction of fitting arrows, and its best to heed these.
I'm sure someone will elaborate.
