Acera Chain Ring alternatives

The M391 crankset is a 104mm BCD, however, as noted in the comments, not any 4 bolt, 104 BCD chainring will be a direct swap fit. The interior portion of the chainring--the 4 areas with the bolt holes that communicate with the crank spider can have differing dimensions and shapes which interferes with the chainring fitting on to the spider. The holes obviously match up, but the shape or amount of material to the inside of the holes doesn't allow a correct fit on to the spider. Many that run into this problem get some use out of a grinder and take off material and shape the ends to get them to fit. The amount of material interfering is just a few millimeters at most typically.

Here is the Shimano Exploded View of the crankset in question. This has the part numbers of the specific chainrings. Also of note is that replacement rings can be obtained from the FC-M430 & FC-M431 cranks and these would fit without any modification. The Acera and equivalent cranksets are relatively inexpensive and since you are changing the cassette and chain (great idea as a worn chain will mess up the new crank and cassette if it can even run on the new stuff without slipping), it may be wise to get a whole new or gently used crankset. At this level, the cost of a new chainring, especially the large one, could be 50% or more of the cost of an entire, equivalent level crank set.