Lucas made spring starters and they've been around for years and I've used them to start cranes/excavators/pumps and site trucks.
I posted on here some time ago of the benefits of having one in the spares locker.
Great to see the idea taken up.
I just can't believe you guys never took out the slug from the shell ! !
I've used them on the Lysander radial aircraft engines and Field Marshall tractors and they worked perfect.