Front is 3.25x19, rear is 3.75x18 (<--- this last one is hard to find, but 4.00x18 works fine.)
You don't have to convert to "modern" sizing. Dunlop, Continental, Metzler, and (just to be thorough) Chen Shin all make the inch sizes that fit the rim properly and have the proper aspect ratio.
For example, I use Metzler Lasertec 3.25 x 19 front and Metzler ME 77 4.00 18 on the rear of my 550Fs. They work quite well! These are tubless tires with tubes installed in them.
Cheers,