Speak to any or all of KHB Photographix, Odyssey Sales (De Vere's successor) and Secondhand Darkroom Supplies and they should be able to supply the runners and other components needed to fix the head to the chassis - all the 504 heads and all the 504 chassis are cross compatible, you just need a few slightly different components.
Regarding timers, as long as they can be wired to the diagram in the manual, they will work. Secondhand Darkroom (who owns the RH Designs label now) can sell you various timers, if for whatever reason the De Vere timer is failing.