Most all the painted plastic (indeed, seemingly ALL the plastic) on the First Gen Venture is ABS.
ABS cement, intended for ABS (black) plumbing pipe, found at your local home center makes for an excellent repair of the fairing parts. When you go to the home center for the ABS, seek out the ABS specific cement, not the PVC/ABS cement that is often offered.
If there are missing chunks of plastic, buy some 1/8" ABS sheets and shape a suitable portion of the sheet to match the missing parts. Finally, use the ABS cement to weld the parts together.
As an alternative to the ABS sheet, try making an ABS putty with MEK (a solvent, also found at the home center) and bits of ABS (black plastic) plumbing pipe or fittings. Placing some ABS bits from the ABS pipe or fitting into a closeable glass jar with some MEK or acetone will turn the ABS bits into a putty after an overnight stewing. Stir the mix periodically and add ABS or MEK as needed to attain the consistency you desire.
Backing the repair with thin window screening or fiberglass cloth offers further strength. After remaking the broken tab, apply a layer of ABS cement and while the cement is still wet, squoosh some of the screening into the wet cement and then brush or swab on some more ABS cement over the screening.
The solvent in the ABS cement evaporates fairly quickly, so don't linger. Also, use in a well ventilated area.