Most of household sewing machines have universal motors. (The rotor's commutator connected serial with stator winding) The motor structure same as drill-machine, and work fine with voltage control, and You have 75% chance for work with normal dimmer with TRIAC output. The TRIAC don't like the inductive load, but i think with small 70W motor no problem.
If You have problem with TRIAC, can change the output two antiparallel SCR-s(thyristor), it's better for inductive load.