A bit of an update...
I cleaned up the working 1GHz model, getting most ofthe cover panels off, removing some of the internal brackets and putting them through the dishwasher. It looks good and doesn't smell any more.
I've got it dual booting with Mac OS Tiger and MorphOS by having them on different hard disks. I wanted to do Leopard but the built-in drive isn't Dual-Layer. It works for everything else though, including burning discs.
The 867 MHz MDD machine, and the bare Mobo, are unresponsive and my last option for the is to try a different PSU but I'm wary to take the one out of the working machine. A friend of a friend does up Macs so I'm getting his opinion. I'm happy to have one working machine for the price I paid though.