I'm pretty sure that's a "standard"-sized pullstart, more common on the big blocks these days, something like this should work. Lots of modern engines have a slightly smaller one.
That could also work with a roto-start, you'd want the "11012" backplate.
What engine does it have? Could you take a picture of the backplate? It's likely there's a pull start out there that'll fit, you just need to find the right one, and that depends mainly on the hole spacing and how far out the one-way hex sticks.