mfgsupply has a tire size chart
Tires
my minibike tire the it goes height/width/rimsize like most industrial tires, I dont have a doodlebug but my minibike has 6 inch rims main thing u will need is correct rim size the rest is width and sidewall height, and they list replacement tires for db as a 6 inch rim tire.
6" Slicks
so find a supplyer that has rounded slicks and your good to go, but youll probably be better off ribbed front and regular slick in rear even tho its squared i never seen any rounded ones in northern or tractor supply, maybe theres some kind of pocketbike that has them, I know i said pocketbike so youll flame me now lol!