As far as I know, if the machine was designed to be ridden on, eg a ride-on mower, you do need rta insurance to use it in a place where public have access (at a show/rally).
I insured my ransomes mg2 with NFU, can’t remember the exact details, but it was a decent public liability cover, theft etc, for £60 for the year.

The issues around insurance came about as a result of changes to the RTA and the designation of certain areas as ‘public places’ this includes shows and rally fields. The machine does not need to be registered, taxed or tested but does need RTA cover. Make sure the insurers know it is for shows and rallies and not for driving on the road. They will need a serial number of the machine so the insurance can be linked to tat particular machine.