For all my MPX-transmitters - even for those from the 1980s - I use lithium-cells and they work well.
6 nc- or nimh-cells provide a nominal voltage of 7.2 V (in peak that might be 8.2 V) 2 lithim-cells go with nominal 7.4 V (in peak 8.4V). So replacement is no problem.
When I started to replace my transmitter batteries I used 2s-lipo-cells first. Later I began to use lion-cells - although they cannot take a very hight load (more that 10 A) they seem to be safer - I don't want to take the batteries out of the transmitter after every day of use. For a transmitter a high current is not needed - usually less than 1 A.
But for charging the batteries should be taken out of the transmitter! Charging via the charging-leads delivered with the transmitter might cause problems! Charger for lithim-cells usually do not work propperly when the battery is still in the transmitter (there is a diode in the transmitter that prevents a charging or better the respond oft the battery to the charger).