I presume you have checked that the heat is down to the brakes dragging and not another cause? Especially given the history of accident damage. Does the wheel spin freely with the caliper removed, without any rumbling, tight spots, etc? Is all the alignment correct?
The next things I'd check are the sliders (the pins that allow the caliper to slide) and the flexible hoses - it's not unknown for these to collapse internally which allows fluid through under pressure to apply the brakes, but provides much more resistance to the return flow, causing the brake to stick.
To replace the pistons you'd need the facilities and skill to properly rebuild the caliper, checking that the bores are correct, replacing all the seals and bleed screws, etc - it's not a job for a beginner, and by the time you've done all that it'll probably cost more than just buying a pair of new or professionally refurbished calipers...
Also, I can't think of any scenario that'd cause enough damage to the pistons to need replacing, without also causing significant damage to the caliper casting, needing at least a rebore (which needs proper machine tools).
Basically, best to replace the whole thing. Don't cut corners with brakes!