If you've got the single oval hole in the seat, then MGB headrests will fit just fine, you can get them from the MG owner's club in a choice of shape, material, and colour:

http://www.mgocspares.co.uk/acatalog/MGOC_SPARES_Headrests_734.html#a32896W_2dR100


Tim H.
1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE