Helps to know exactly what RD that is. The 600 or other names they put on them don't necessarily mean that any with that logo/name have the same specifications. Shimano usually has the part number stamped on the back of the parallelogram. But for that old, it might be elsewhere. It'll start out as RD- along with some numbers and maybe alphabets too.

But that looks like a rd-6200. So you should be good for any freewheel that has 28 or less teeth as the max low sprocket. As long as you aren't changing other stuff, you should be good with a 14-28. Though you probably will need a chain also.

That old Regina FW might have a non-standard hub attachment that you won't find on todays FW's. So you might also need a new wheel or at least a new hub. Depending on what you wish to do.