I have some spare cam shafts but none for R60es. Some may be small seal. And I don't have them here but next time I visit my stuff in storage I'll try to remember to look at them. I've never heard of this two piece cam shaft deal before. Of course it may of just not come up?
I don't think you can use the larger engine cam shaft. I know it doesn't work the other way and suspect it won't work for you either. So you need the correct part. Unless maybe a large seal number could be used with a different timing cover. But It sounds like too much change anyway.
If these two pieces are pressed together and they have merely worked loose it sounds to me like they could be made to work again. I'd even say weld them together. But spare parts should be readily available.