West Marine has a kit with stencils and a paint that's supposed to last.  In practice, I find that it tends to get abraded away after maybe 4 seasons and needs a touch up now and then.
Wonder if a permanent marker would do better (but haven't tried).
 

Have you tried any of the Krylon exterior paints?  I have good luck with them on outdoor furniture. But you would need to test for compatibility with the PVC.
I used the magic marker on a plastic panel but it faded in less than a season.  My ID letters even faded when they were left in the sun.
 

I painted the registration on my dinghy (a west marine/zodiac 8' inflatable) using the kit from west marine. It did the job well enough I suppose, but I regret painting it. I see plenty of dinghies with their reg numbers on a plate affixed to the forward corner(s) of the dinghy. If you can get away with it, that's the way to go.

The real trouble I had was getting the registration sticker to stay on. I painted a black square and stuck the sticker to it near the registration, but it kept peeling up so I ended up taking it off as I feared losing it. Not a big deal for me as I trailer the dinghy (long story), but could be a big deal if left on a dinghy dock.

Here's the kit I used... If I were to do it over I would affix stickers to plates hanging from the dinghy.

https://www.westmarine.com/buy/west-marine--lettering-numbering-stencil-...

Mike McCarron
Under Agreement for a 1999 C36 MKII
Phinneys Harbor, MA

https://duplicolor.com/product/vinyl-and-fabric-coating

Used Dupli Color Vynal and Fabric approximately 15 years ago letters & numbers stil very legible

I used the preprinted numbers that both WM & Defender sell. They look professional unlike the stencils I’ve used. They apply like a patch so stick very tightly.

I needed some way to cover the old numbers with the new. 

Port Side NumbersAfter looking at several ways to cover the old numbers I finally settled on a product from BoatNumberPlate.com (can be purchased through West Marine and other retailers).  These are flexible plates with the registration number imprinted.  The plates can be made out of either Hypalon or PVC.  They are as flexible as the inflation tubes on the dinghy and should last through multiple deflations and inflations.
In Florida there is only one sticker and it is affixted to the port side.  So the port side plate has room for that sticker.
When I applied the Maryland numbers on the dinghy, my spacing was a little wider than that on the plates, so the plates do not entirely cover the old registration numbers.  The port side plate, with the place for the sticker, is long enough to cover the old registration as shown above.
However, the starboard plate, without the sticker, leaves a portion of the old registration visible.
I'll probably just let those old number fade away.

I needed some way to cover the old numbers with the new. 

Port Side NumbersAfter looking at several ways to cover the old numbers I finally settled on a product from BoatNumberPlate.com (can be purchased through West Marine and other retailers).  These are flexible plates with the registration number imprinted.  The plates can be made out of either Hypalon or PVC.  They are as flexible as the inflation tubes on the dinghy and should last through multiple deflations and inflations.
In Florida there is only one sticker and it is affixted to the port side.  So the port side plate has room for that sticker.
When I applied the Maryland numbers on the dinghy, my spacing was a little wider than that on the plates, so the plates do not entirely cover the old registration numbers.  The port side plate, with the place for the sticker, is long enough to cover the old registration as shown above.
However, the starboard plate, without the sticker, leaves a portion of the old registration visible.
I'll probably just let those old number fade away.

I used a product from Boat Number Plate https://boatnumberplate.com/   You send them your registration numbers and the state and they return you a printed 'plate' that you glue to the dinghy.  There is a spot for annual/biannual sticker and any state specific stickers.  I put mine on over 10 years ago and have had no problems.  I can't remember if the kit includes glue, but any glue that is used for patching your dinghy will work. It's a little pricey~ $45.00, but you do it once and forget about it.