Sometimes I think IAME hired a British electrical engineer whose prior experience was with Lucas (“the prince of darkness”)
External starter is your best option. Option 2 is bypass the box by running a + line direct from the battery to the starter, i.e., a jumper cable. If that works, splice a high-current switch into the line and mount it permanently.
Option 3 is trade your leopard in for a new motor (welcome to karting, where even cheap racing isn’t as cheap as you’d like).