Kaweco refills are simple in that they are either a rollerball or ballpoint pen refill, but within those types is more selection, which depends on the specific pen you have. Our advice would be to compare the old refill with the product photo and description to try to determine which one you need, and failing that get in touch with us for more advice.
Kaweco D1 Ballpoint Pen Refill - Medium - Red (Pack of 3)
High-quality D1 ballpoint pen refills from Kaweco, suitable for all Kaweco Sport ballpoint pens.