How would one find out who owns the copyrights for cancelled games like Arcadia VI and Scarabeus (and/or get a license to remake and sell them)?

Regardless of if you know who has the rights to them, it doesn't mean they have the source files, or any copies on hand. This has, unfortunately, often been the case. I think Scarabeus was dumped and you may already know that. I don't know much about the dumped status of either, honestly.
 

I'm not familiar with the prototypes, but the information I can find is that they were both developed by Source (Or rather Source Research & Development LTD.)

http://199.101.98.242/media/image...
https://tcrf.net/Category:Games_d...
https://tcrf.net/Arcadia_VI...

Looking up info on them, I've found this:
http://www.datalog.co.uk/browse/d...

This is probably your best bet to contact them. It seems that they were recently dissolved as a company, but even if it's abandon or orphanware, they could still hold the rights to those games. 

Both original ROMs I referenced were dumped, though that's not much of an issue, since the remakes would likely be on PC.

I'll see what I can do, then.