Method:
29.97fps MPEG-2 source
-> decode
-> IVTC filter
-> re-encode
23.976 MPEG-2 output
Re-encoding a lossy source is never ideal, but keep in mind that encoding 23.976 progressive fps is more efficient than 29.97 hard-telecined fps (effectively, there'll be 20% fewer frames to encode). With a good MPEG-2 encoder, losses should be minimal.