Do you know the make of dynastart it should have such as Delco, Bosch, etc?
I’ve had experience of finding dynastarts for ourselves and the ones on some of our own machines are specific to the mounting bracket on the engine so those dimensions may be of use when searching for one.
Do you have the electrics on the tractor already or can the electrics be sorted, altered and wired up after the dynastart has been sourced and fitted?