You push! You can actually do it with your hand from inside the car on flat ground it is that light. With a bit of careful driving you can avoid needing to do it most of the time.
Some people fit an electric reverse. Others a mechanical one. These are generally unreliable and add lots of weight. At the end of the day, the car is designed to be a track car that is road legal. Until they change the law (Which is happening end of April this year) a reverse ability is not needed for the SVA (Single Vehicle Approval) Test. I will be on the road by then so will avoid needing one.
[I believe most kit companies are designing a temporary reverse system to be fitted for the new type SVA, called IVA (Individual VA), which will be removed after the test. The car is subject to a normal MOT under the same rules as a brand new production car once it had passed SVA/IVA and therefore does not require a reverse gear.]
