I think it mainly depends on two things:
1. what exactly do you want to achieve?
2. what is your budget?
On defining the objective, it's not clear to me what you really want
A. do you want to convert to a quill stem specifically?
B. do you just want to convert to an easily adjustible stem because you expect to move it frequently?
C. do you want to get your bars higher and be done with it?
If it's A then you need to change the existing fork, and buy a headset and a stem and rebuild your cockpit. If it's B, maybe you just want a longer steertube, since the previous owner cut it shorter than you wanted. If it's C, maybe you could also get your bars up with a different stem.
The list of things I imagine are possible include
-ride it the way it is (free!)
-get a custom stem ($250)
-find a different stock stem ($ ??)
-get a fork threaded, buy a new quill stem, and a headset and labor ($500 ?)
-get a new fork, buy a new quill stem, and a headset and labor, and maybe paint ($1000 ?)
-get a longer steertube fork but keep all other parts ($400 ?)
There are probably other choices depending on what the real objective is.
Bill Lindsay
El Cerrito, CA