If there is any visible damage to the pistons, then scrap them - if they were cracked, they most likely would have failed spectacularly by now (cracked, melted, severe skirt wear or valve impact). If not, and the rods are straight (cast rods will bend before cracking), it's pretty cheap to have a machine shop magnaflux the rods. ($25-40). If you have the tools (micrometer and T-gauge), it's not too hard to check the big-end bores.