Is it safe to run a 7.5 volt electronic on a slightly higher or lower power amount?
The OEM adapter (AD-1) is a non-regulated type so it is not very critical. You can get an (claimed) equivalent from various eBay sellers as a switchmode supply, but they don't tend to have UL/CSA markings. You can get used Casio ones, but they are quite expensive.
Here is the manual section from Casio's manual for the SA-75:

From another drawing in the Casio manual, the center pin is NEGATIVE (as is LESS common). Connecting the wrong polarity of adapter is likely to cause damage.

From photos of the OEM adapter, it should be at least 600mA (more is okay).
There are two common sizes of the center pin - 2.1mm and 2.5mm. Based on seller's specifications, it appears that the right size is 2.5mm. It would be good to try to confirm by measurement of the male pin diameter.
Legitimate electronics parts distributors such as Digikey have various properly approved adapters, for example Triad WSU075-1000-R13, which looks appropriate, for barely more cost than the sketchy ones.