Probably any retail speakers that have the power handling for a 100w amp would be an upgrade to the tone compared to the Marshall MG stock speakers.
A pair of V30s (60w ea.) will be adequate, but that's about as low as you want - don't pop a pair of greenbacks in there...
Also, make sure you match the impedance of the existing speakers.
Higher impedance would be ok, but lower would be bad for the amp.