For a bike like this one the limit of tire width is usually at the chain stays. 27 inch vs 700c is only a 4mm difference in major radius so it makes basically no difference in available width, less than a mm. If you do have the width available, Kenda has a knobby 27 x 1-3/8 tire. They aren't racing tires but they will get you down a trail. You can see some C&V conversions where they increase the crimp to get a few more mm, usually for a 650b tire, which also moves the fattest part back nearly an inch.

If this bike is rear spaced for a 120mm wheel hub then that is ... more project