First thing to do is to ascertain where the play is - if it's both crank arms moving together, and relative to the frame, then it's likely that the bottom bracket bearings have gone, which will introduce play at the spindle, in which case replacing the BB alone may be enough - take the old one out and find an exact match using the information stamped/printed on it.

However, if the play is between the crank arms and the spindle itself, then it may be that the crank/spindle interface has become damaged/worn, and it's more likely that you'll need to replace the crankset as well.

Assuming you're happy with the existing gearing, and just want to replace like for like, then the Acera M361 crankset would be a suitable replacement. You'll need to match it with a bottom bracket with "English" threads and a 122.5mm spindle (spindle length is dictated by the crankset, not the frame). You'll also need to check the BB shell width (the bit of the frame the BB sits within) - it will be either 68mm or 73mm, and will be written on the body of the old BB, or you can measure it directly with a ruler. Then search for UN55 and choose the correct one from the options.