@medoramas
Nice 29 inch bike you have!
The RST fork has a 1 1/8″ steerer.
Pretty sure the frame headtube is 44 mm then. You can roughly check the outside diameter top and bottom. If they are in the range of 45.5 mm diameter (outside) or so it’s 44 mm inside.
In this case: you are able to fit a tapered fork with the mentioned headset (or similar ones).
In the worst case: the 44 mm inside “machining” of the headset needs to be 9 mm or so. Normally it’s the case.
If not you might have to grind a mm or so off from the aluminium part of the new headset part which goes into the tube. But that’s rare.
With tapered forks you have much more choice now!
If you get only a low priced BOOST fork you also need a new front wheel. Cheapest are (in sale) around 75 Euros. Old model Mavic Crossride 29 inch boost for example.
Less trouble you find a fork with your axle dimensions (quick release).
Additional to fork and headset you need a 1 1/8″ steerer spacer set (aluminium, 6 bucks or so). Couple spacers on top of the stem and you don’t need to cut the fork steerer tube so short. Bigger chances that you are able to use the fork then for an other bike or that you get a better price when selling the fork later (there are bikes around with really long headtubes…).
Hey – your bike gets a new life with a new fork!
Old fork has 100 mm?
Buy a new one with 120 or 130 mm …
More sag. More fun. Plush.