Modulating frequency of a crystal oscillator circuit
The term for what you're looking for is probably a VCXO. These exist in commercial forms, but one common arrangement is to use a variable capacitance in parallel with the crystal as follows:

In practice, varicaps can be used; they are a diode, constructed to have a capacitance that varies with the DC bias across it. One realization described in this source uses a pair, back-to-back, to create a capacitance that varies with an applied control voltage:

You should be able to use an off-the-shelf through-hole varicap; in a pinch, a MOSFET with source tied to drain can act as a variable capacitance between the gate and source+drain terminals:

There is a downside to this design however -- the additional capacitance reduces the quality-factor of the oscillator, and may introduce extra phase noise. The frequency deviation is also limited by how far the crystal itself can be pulled from its natural frequency. Instead, you may want to consider a non-crystal VCO which generates a low or intermediate frequency, and a mixer that upconverts this to your desired carrier frequency (your crystal oscillator would be the local oscillator for this arrangement).