[morzh]

Even though it will probably work in the majority of cases, I would prefer to replace like with like.

The capacitance of a flat cap (which is what it is in this case - a flat cap) is

C = (Permittivity * Area)/ Distance

The permittivity is the characteristics of dielectric materials and for Mica it is 3 - 6 while for Plexiglas (clear plastic) is 3.4.
http://hyperphysics.phy-astr.gsu.edu/hba.../diel.html


So your cap using mica could potentially have twice the capacitance of the same geometry cap using plastic.
This might still work as the cap is adjustable.
Mica will also not warp due to heat which may or may not be of significance depending on what the radio is like.


Of course plastic is easier to work with; drilling hole in mica, one has to be careful not to ruin the sheet.