Lots of sources for forged 413 pistons, you just need to be willing to pay for them. Diamond, Wiseco, JE, Ross, etc can all make 413 pistons. It is just a tall 383 piston, or a small diameter 440 piston, either way it is not really a big deal. There isn't anything special a 413, standard valve angles and pin size. Not sure what kind of rings are available in that bore size so that might be an issue. There are a lot of good rings available at 4.250 but that is a lot of material to remove. If you look around you can probably find SB Chevy rings in the 4.210 range.

Forged flat tops will probably run about $700 a set with pins. You just need to know how what to ask for. If it was me I'd start with Diamond Racing and see what they say. You'll need to know what bore size you're shooting for before you'll make much headway though. A 0.030 over 413 is an oddball size and there isn't going to be much more than low speed truck rings avaiable at that size.