yeah sprocket size does matter on a T Bird the freewheel is is on the back and is like 18 teeth and the front cog is like 22 teath.
Your best bet is to buy a new crank pair and a new freewheel with a good engagement such as 72 upwards and but a screw on sprocket on the back, i would advise running 12 on the back and 18 on the front.