Forks are somewhat universal, so you don't need to buy another RST.
The key elements are wheel size, ie. 26" brake type, ie/ V or disc, and the diameter and length of the steerer tube.
So, start by confirming the steerer (top of the fork) diameter, which is probably 1-1/8". You can measure the ID of the stem, and don't have to be precise, since it's either 1", 1-1/8" or 1-1/4" (rare).
Then the length of the tube is critical, though most new forks are sold very long and cut to size by the owner. So, a fork for 26" mtn wheels, set up for V-brakes and the right steerer and you're good to go.
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