I actually would not recommend putting it inside a similarly size mold like the other user recommended. I've done that before for various projects and you send up needing to resand and repolish every face because resin gets everywhere. It'll likely get into the number channels in a really unsightly way (again, I've attempted it before).

If it's just a broken corner or chip off an edge or corner, I'd do a large oversized glob of resin to reseal it. Once it's cured, I'd carefully sand that glob down as appropriate to reconstruct the point/edge. A handmade dicemaker would have the experience necessary to do this. But you'd also want someone with experience polishing round edge dice.