Typicall fixed fork only come with single speed.
So you are stuck with:
- Replace a suspension for a fixed
- Add gears to a single speed
Don't do it unless you are going to get a good carbon fork and that is $300+. It is nice for weight and does not wear out but it is not cheaper than low to mid range shock.
I think it is more economical to add gears to a single speed.
If you can keep the wheel.
SS with an eccentric bb they typically use a standard wheel and free hub.
I think the Salsa alternate dropout also take a a standard wheel and free hub.
With most ss you can can add gears.
You can get some used gears from someone that upgraded for decent price.
Or ride a hard tail until you wear out the shock and then decide.
How about shoot down the theory you need gears?
A single speed is more versatile than you would think.
I would not go single speed to cheap it.
If you do, go with nice carbon fork.
And some nice tubeless wheels.
With a suspension the tubeless gives you a bit more give.
And you can go 2.4 in and still be less weight than a shock.
You get more ground clearance.
And go with clip (e.g. spd) for more hop and two leg pull for when you stall out.
You can change out the rear cog from ride to ride.
Typical gearing is 2:1 (32 16) and I have 18 and 20.
It climbs OK - where you have trouble is if you stall out on like rock or root.
It forces you to be a better rider.
The solid frame and low weight is an efficient climber.
And you can add gears. They typically come with a regular wheel with a conversion kit and a derailleur hanger.
I got one used cheap to learn technique and found I really enjoy it.
This is a bike I picked up used for $850 - I bought it basically for the fork.
There is some discussion on conversion:
This is multi speed that was converted to SS
This is a mountain bike purchase as a SS
As you can see the wheel hub is the same.
The difference is the conversion has the chain tensioner.
This designed SS has an eccentric bb to adjust chain tension.