What's the best way to repair a car roof after removing a sun roof?

6 months back I had a sunroof removed & the roof repaired, but the filler over the patch in the centre of the roof has now cracked, with a few adjacent cracks running off the sides of the main crack. The crack is approx 45cm long. Before I let the guy who done the body work attempt another repair, I'd like to know how it should be done to ensure it doesn't crack again.
Thanks for the advice guys. The car is a 2 door 1975 LA lancer, of which there aren't too many on the road these days, hence finding a good donor roof will be near impossible. In order to keep the car near original, I'd rather not put in another sun roof, unless it's my last option.

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