From the NASA CCRs:
11.4.7 Roll Bars
All open cars should have a roll bar installed to help protect the occupant(s) from injury during a roll-over. The
main hoop shall be one continuous piece with smooth Mandrel bends with no evidence of crimping or wall
failure. All welds should be of the highest possible quality, with full penetration [Ref
15.6.15)]. All cars with roll
bars are required to have adequate roll bar padding per CCR section #15.6.4. In cases where the driver’s head
may come in contact with the roll bar should the seatback fail, a seatback brace is required in conformance with
section #15.6.21. Acceptable roll bars include, but are not limited to, the following:
Which means, if it doesn't have adequate roll over protection, the top must be in place.