The gap in the MDF is where the top part attaches to the base of the top part (it has a small piece at the base that fits the gap)As above, it might be cheaper to replace the whole lot.
The last time that I asked my local timber yard to replicate some missing picture rail, they warned me that it would cost £300 just to have the spindle moulder cutters made.
As an aside, why are there gaps in the MDF?
My local yard is £40 to set the cutter then X amount per metre to run off.
Cutter is just a allen key and adjust to get the shape I think