Cracked pistons are usually caused by heat stress, which is common on the TD engines. You drain the oil and remove the sump, then remove the big end cap nuts and push the piston/conrod assembly up the bore. Piston kits are approximately £40 each and come with rings already fitted, piston pin, and circlips. Big end caps are matched to their respective conrods, so make sure you keep them in order.
Les.