Problem with Petrol Classic 35S mower?

I have a Qualcast 35S circa 1996 mower which I am currently renovating. Having bought and fitted a scarifier cassette I scarified one lawn and then an awful mechanical screeching noise came from the drive area. I removed the side casing and found that the pink drive transfer cog spindle had come detached from its mounting plate. This plate which is welded to the casing also supports the cylinder drive pulley. Any ideas for rectification as I dont' want to dump the machine having spent so much time and money on it?