If you unscrew those screws on the top cover of the drive you'll let the magic smoke out. Don't do it.
5.25 inch drives like that are secured in the same way as the more modern varieties, meaning it either has four screws run into the sides of it (about a half inch away from the bottom), or it's secured with screws going up from the baseplate into vertical holes in the bottom. I've never seen an SC20 enclosure in the flesh before, but I'd try flipping it over and seeing if there are four screw heads arranged in an outline around where the drive is. If that's not the case the enclosure may have a "false bottom" shelf the drive is sitting on, and you'll have to figure out how the Chinese puzzle goes together.
I'll just mention this "because", but... computer surplus places still often carry generic SCSI drive cases in both 3.5 and 5.25 inch sizes. (half-height 5.25 enclosures housing CD-ROM drives are particularly common. You can usually easily adapt those to hold a hard disk with a suitable adapter sled. You might also find cases with tape drives in them that will do.) You might want to leave your antique as it is and get a new enclosure for your modern drive.
As another aside, I have to say I'm impressed to see a working Seagate ST-225. The MFM version was dirt common in PCs... almost thirty years ago.