Yes you will need RTA insurance on the rally field, you will not need to register or tax the MG5, insurance will be against machine serial number rather than registration number. A number of insurance companies can provide suitable cover approx £30-£40 a year will also cover for theft as well in most cases. I used the NFU when I had an MG2. If the insurance company tells you it needs to be registered try somewhere else as they obviously do not know what they are talking about.