I recently got an inexpensive P-bass clone. It's a "Sanatoga", a company that I think is owned by Jay Turser. It looks, plays and feels great. I don't need another fretted bass so I was thinking of getting a fretless neck to replace the original.

So.... I'm going to assume that any Fender licensed neck would fit OK. What else, in terms of problems, could I be looking at? I've never done this kind of work, and since the bass cost me very little I thought I'd use it to learn a few things.

Any help, suggestions, caveats etc etc are a big help.

thanks in advance.

Don't bet on a Licensed Fender neck bolting straight in. A LOT of clones use non standard screw holes.

Have you though about converting the existing neck to a fretless neck? It's a pretty painless conversion, just pull the existing frets, fill the gaps with suitable filler, sand and your done, plus it will have the advantag of having fingering guides for locating your fingers.

I wouldn't worry about the screw holes not matching up. i would always expect to have to blank off with wood dowels and redrill for a replacement or neck transplant anyway.
The real question is the neck pocket rout - right dimensions. This is something most clones are matching on (but not always) - sometimes you have to expect a little shim or fillet to be added to get the best possible snug fit, but it's not a big job and it's good experience at helping set up a good action on the neck. Go for it.

Have you though about converting the existing neck to a fretless neck? It's a pretty painless conversion, just pull the existing frets, fill the gaps with suitable filler, sand and your done, plus it will have the advantag of having fingering guides for locating your fingers.

Yea, I did that with a Washburn a couple years back to see if wanted to get into fretless. I've been playing fretless ever since, so I really don't need the lines.

I wouldn't worry about the screw holes not matching up. i would always expect to have to blank off with wood dowels and redrill for a replacement or neck transplant anyway.
The real question is the neck pocket rout - right dimensions. This is something most clones are matching on (but not always) - sometimes you have to expect a little shim or fillet to be added to get the best possible snug fit, but it's not a big job and it's good experience at helping set up a good action on the neck. Go for it.

Most of the new necks I've seen don't have any holes drilled so the matching of the holes is not going to be an issue.

The neck pocket is something that I'm worried about. If the pocket is too large I'll use some shims, if it's too small, how do I cut out wood without messing the whole thing up. And... how can I be sure that the neck is on straight and that the strings from nut to saddle will be in the correct place?

As stated above, if you order a neck make sure you measure the neck pocket and double check everything. IMO, Warmoth makes the best replacement neck on the market. They are better than factory Fender necks.
That being said, I'm a big do-it-yourselfer. Pull those frets out or replace the fretboard. You can order fretboard blanks and glue one on your existing neck. You will have to get a hot knife and remove the old fret board. Not the easiest thing to do, but not as daunting as it may sound. Have fun!