The Ohlins HO-204 shock fits all RC51s.
I would also search for a used HO-204 up for sale as well.
You could grab the shock for much cheaper than new.
And for the money saved, you could have it serviced and sprung properly to your weight.
Elka is a fine shock, made in Canada.
However, due to market penetration, you'll probably find more Ohlins service locations than Elka.
Also, Ohlins USA does a quick turn-around on the shocks as well and are very friendly.
However, from what I understand, the Elka is more user serviceable than Ohlins, and doesn't require special tools, other than a spring compressor and nitrogen source.
Watched an example of shock servicing on the Internet that used an Elka as an example, and it was very straight forward.
I heard the Penske shock is also user serviceable in a similar manner.