Harp check out the SWR web site...... they list a few options there. Pickup's?? What is wrong with the Basslines? What are you looking for... Active/Passive combination...EQ...? Take a look into the Bartolini web site..... they have a pretty good shopping list there also. They break it into type/models making it pretty easy to select.
Now having said this, unless you are skilled with your hands and have the tools.... I'd leave this work to your local luthier. This person, if knowledgeable and I don't see why they wouldn't be, will be able give some great advice on your upgrade questions. Also ask about 'shield' the electronic cavities and channels.. this is fairly inexpensive and does add to the quality of your over all tone.
This is IMHO of course.
MO