Bottom line £400 to get it sorted. I would send it to a specialist like Board Bros. in St. John's hill in south London. The lifters are cheap enough but a cam is £500 easy. They may be able to make you one for less or at the very least check it and make it good again (they are very good) either way it's bucks. If you can find one on eBay you could be in luck but it will still set you back the initial £400 hundred as they will have to size up and space all the lifters to fit (not an easy job for a hydraulic one)
Mechanical lifters are good in some respects and you can usually tweak them to get better results than a hydraulic. That's no help to you as you are stuck with what you have but it's helpful to know for the future.