Obviously, there's a technical side and a legal side, but there's also an aesthetic side.
Technically you can do it. There's a technology called photogrammetry that lets you take a series of ordinary digital pictures (dozens), throw them into the software, it crunches them and spits out a "mesh", a 3D scan. There is capable software now that is free, produced by a collaboration of universities. You'd have to learn the software, but from videos I've seen it's not too hard. You can then pass that to a 3D printer and print it at whatever size you want, make it 2mm high, make it 300mm high, whatever the printer will do.
Legally, you're going to have people argue no matter what you say. It's generally accepted that you can copy your vinyl LP onto cassette so you can listen to it on your WalkMan. You could argue you're doing the same thing here *provided* you only print one copy. You're getting the same use, you're not costing them a sale. The counterargument is that by changing the size you have "mutilated" someone else's work, I think that's the legal word for altering copyrighted material, and by changing the size you have changed the use. An expert would have to speculate for us which way a given court might rule. Certainly if you make more than one copy you have cost the original producer some sales and they can sue.
And of course, the .OBJ file (I think that's what the photogrammetry software would give you, but it might be .STL) is of dubious provenance. Like a photograph of a statue, it's a creative work entirely derivative of another creative work. If you were to allow that to escape you would be in murky waters indeed.
Aesthetically, except in the case of modern naval ships virtually all miniatures are distorted so they look right at various scales. If you simply shrink down a P-51 or a human being you will get an unsatisfying miniature. You need to highlight distinctive features, oversize some details, etc. Otherwise everything looks bland and alike. You will very often find in 3D printing that something that looks great in one scale looks wrong in another. It's possible the style of the sculptor involved translates well from 54mm to 28mm. You might also find that shrinking it makes things look squat, muddied, bulbous, inflated, deflated, globby, who knows. One miniature may convert, another may become loathsome. You can't tell without trying.
3D printing and casting are also quite different technologies with different strengths and weaknesses. Some things look great 3D printed, other things look awful. Surf the net, you'll see 3D printed figures that look terrible and some that look awesome. Someone with experience could look at the figure and take a guess at whether it would be worthwhile to 3D print it, but you might not know for sure until you see the print.
It's also possible that the 3D scan would have to be "cut" and printed as several pieces. That can improve the results in several ways.
In any case, you'd have to learn photogrammetry and 3D printing. I'm assuming from the way the question is stated those would be new to you. It would be quite a project, and if sounds like fun it would probably be fun but if it doesn't sound like fun I'm pretty sure you wouldn't get all the way through. There would be a lot of tweaking along the way.
Best path forward is probably to contact the manufacturer, respectfully. They may have an interest in pursuing this in some manner.
