I don't know so much about 'limited alterations'...

Manifolding and carburetion were free (though injection wasn't allowed), from memory the stroke had to be standard but they could be bored or sleeved, any form of modification to the head was allowed as long as you didn't add material, camshafts were free, all reciprocating and rotating parts were free.

I think it started in Italy before 1959, too. It went big Internationally in 1960.

Count Johnny Lurani was behind it as I recall.