They might be acceptable from a formal language perspective, but it is simply not used in the context of financial instruments.
For example, a google search for “swap site:ft.com” gives 20,600 results. “swop site:ft.com” provides 50 results, many of which appear to be using “swop” in the non financial sense.
“swap site:bloomberg.com” provides 44,700 hits compared to 48 with the alternative spelling.
A google trends comparison of the usage in total (not even restricted to financial terms) shows this fairly categorical perspective.
The UK Telegraph’s style guide prefers swap over swop.
Finally, and most fundamentally, the ISDA (International Swaps and Derivatives Association) uses “swap” exclusively.
So at some point it just becomes silly to use a word that nobody else uses. The use of “swop” rather than “swap” to reflect financial instruments simply shows up the distance from the actual instruments.