This is the largest digital version I've been able to find, but it's missing much of the original cover (and has noticeable jpeg artifacts): https://i.lensdump.com/i/WDIi91.jpg

There's a version of nearly identical size with fewer artifacts compared to the uploaded version, but the color is washed out: https://i.lensdump.com/i/W13t2i.jpg
And a smaller version with virtually no artifacts: https://i.lensdump.com/i/W131Co.png

Best way to do this would probably be to enlarge the parts missing from the scanned cover, optimize them, and attach them to the digital version (or just work off scans upscaled in Waifu2x.)

Here's a couple of attempts at enlarging some scans in Waifu2x- the best outcome is the first, but the colors are pretty off: https://i.lensdump.com/i/W13jfK.png
https://i.lensdump.com/i/W13I0Z.png

Here's the original of the first resize: https://i.lensdump.com/i/W13iKc.jpg
And here is a mostly complete digital version: https://i.lensdump.com/i/W13gYm.jpg