Link not working. So what would be a good/safe one to try?
Maybe eBay has some.

Link works fine.

Look at how much torque the included servo produces, decide how much more you need, buy an appropriate servo.

Try moving the guide closer to the spool.  The idea is to get the guide as far as you can from the point where the filament is bending.  Like a lever, there is less force required further from the pivot.  Another thing you can play with is running the tube through only one of the holes, where the filament goes in, and letting the other end float free.  If it is less constrained, it might not be able to pull back on the guide as much.

There are tons that you can look at on that page. Now on to the issues with replacing the one that came with your Filawinder... Whatever you'd buy would need to be within the power range the Filawinder can handle or you'll either burn the Filawinders board or be unable to get the torque the servo "could" put out.

Next up is the possibility of needing to customize the software. For instance if you went with a gear reduction servo, you might be able to stay within the power limits but would need to alter the software to get the servo arm to be in the position that the winder thinks it is.

If you can overcome one or both issues, then I'm sure you can upgrade your Filawinder.

Thanks !