The bike in the question is not what is considered as "track drop outs".
For that bike, depending on the rear wheel spacing, it should be relatively easy to convert back to geared.
I would suggest, leaving the front as a 1x system. This is subjective, but I would just get a relatively moderate ring on the frontm between 40t to 46t depending on how hilly the area is.
If the rear dropout is 126mm, then you could either get a wheel to suit, this is an old standard, or you could get a newer 130mm wheel, although how well this works, depends on a few factors.
You could get get a mech hanger such as this
![enter image description here](
Which sits at the rear of the dropout.
Alternatively, you could go down the internal geared route.
Both options have their own pros and cons.