Hello Vive le Sport
Try once to put shims on the top of the cam gear and check how much freeplay you have.
Your aluminium piece is very nice, but i think ther is a major extension in lenght when it gets hot. Maybe you have to much play or you have no play and it seizes the cam gear when hot.
Also you have another problem with your piece, the cam gear is not guided on the top, maybe it turns not round and start to bounce.
What height has your cylinder head and wich thicknes has the gasket?
You could also lean from a friend a distributor to try and you let it turn just without function and check the wear.
What engine oil do you use?
Regards
Daniel