About the oring:
Unikorn seems to ship with a 50 Duro Silicone Oring, probably a 262 or 261 (I bought a 263, which is slightly bigger than stock, and a 259 which is much smaller than stock).
Replacements can be 50 or 70 Duro, 262 size or a little smaller. I also bought a 259 50 Duro Silicone oring; it tested fine. (It did stretch more but I didn't notice a difference in feel.)
Oring vs xring doesn't seem to be significant.
Plate probably contributes more to feel than what kind of oring. (Though having an oring mount obviously contributes significantly.). My plate was the stock brass one.
Note that higher Durometer ("Duro") is firmer. Also note Durometer has different scales. 90A is softer than 50D (the bigger letter is a harder scale).
90A Duro doesn't stretch much, so I'd avoid that.
Note on the sizes: I'm using the "dash" number, which is an American Industry thing (well, I mean, it's not metric). Actual size is 1/8in cross section, 7in Inner Diameter (or so).
Prices for these a
were all around $1 / oring. $3 / xring from theoringstore.com as well as oringsandmore.com.
This is helpful to understand the sizes I was mentioning
https://www.oringsandmore.com/oring-size-chart/