Burnt pistons

Finally got my engine out and started to tear it down. Found 3 pistons burnt not through but you can tell the crown has burnt spots all over them. My question is I'm wanting to build build a 500 to 600 hp engine maybe adding a little nitrous later. Should I rebuild this engine, replace the 3 pistons ( I haven't measured the bore yet but there's no wear ridge or damage visible and the mileage is only 189k) or try to find a good used long block? It's a little hard to find a decent used engine out here for less than $1500 - $2000 then I wouldn't know exactly what id be getting without tearing into it. Anyhow advice is appreciated.

Billet rods and block fill are needed for 600 hp

If you are shooting for decent power, and need pistons anyway, why not put some delipped and coated ones in? 600 hp is a bunch on a 7.3, so id be overbuilding personally.