An advantage that has been found in running internal combustion engines with Hydrogen, instead of in fuel cells, is that the hydrogen does not have to be quite as pure to work in an internal combustion engine as in one with fuel cells. Another advantage you see when using internal combustion engines is the possibility to switch between different types of fuel. Furthermore, using already proven technology will keep costs down at the factories.

Whether or not there will be conversion kits available in the future is a question that no one can answer at the moment, but it would be more environmentally friendly to switch over from fossil fuels to fossil-free fuel as soon as possible.