I have 2 of him... sadly it's one of the factions Rackham never got to finish filling out sculpts with... more mounted cadwallon mounted sergeants and knights would have been great... I mostly use my rackham stuff for dragon rampant games... I haven't painted my knight yet, but I like your idea of head and horse swaps...
I had asked about this on
Confrontation Haven too, but no conclusion - maybe ask the sculptor? I'm tempted to saw off that part...
I don't like that the lance arm is connected at the elbow, as it's difficult to pin, but this appears to be the case with other Rackham figures.
If standing upright, this figure would be around 40mm, yet its limbs have the same thickness as a 25-30mm miniature. Another thing about later Rackham figures is the sculptor ignoring the fact that armor and padding underneath adds some bulk. Without armor this figure would have bony arms! At least the helmeted head looks reasonable, while on others a bareheaded figure look like he/she has microcephaly.
Since the figure's limbs are closer to 30/32mm, I'd suggest parts from the Perry
WR 40 Mounted Men at Arms 1450-1500 and
WR 50 Foot Knights 1450-1500 boxes: the hands are the same size, but the arms are a little short, requiring some clever converting of the metal parts and some of the helmeted heads are also compatible.
Regarding horses, I'd suggest the Perry
B 29 Wars of the Roses/EA horses. These are the same size as the Cadwallon Knight's horse and come with sculpted saddles. Oddly enough, the metal horse's head is a little thicker, due to the cloth barding and chamfron - the sculptor got that right!
Steel Fist Minatures' horses are also compatible, but you'll have to add a saddle and don't know if these are available separately for sale - I'd also enquire about separate plumes. If I could source some very thin plastic sheets, I could curve and attach these with plastic cement to the horses for cloth barding and bought a mail texture stamp from
Green Stuff World.