Same thing happened to my engine, 500kms in.
New piston, valves, head and a new idler bolt later. Was rebuilt and has been going strong.
If you eventually get a new stronger replacement, i would be buying one off you in a second if i or anyone i knew was building an RB.
As the result of a snapped $15 bolt creates a whole word of heartache
And Daniel im sure you have done a few timing belts, but i think the tension has a part to play with these bolt failures also.