Hi Tor.
Nothing like a free tool . Especially a lathe so you can be much more diversified along with you other woodworking hobbies.
Here is a parts list of your lathe .
http://www.owwm.com/files/PDF/Dremel/700708revparts.pdf
A missing tailsstock center should not be hard to find. Do you know if it is a #1 or # 2 morse taper on that lathe ? Or does it even use the morse taper principal or some other way of holding a pen mandrel?
Is this what yours looks like ?
I have seen a few of them on sale in ebay and some other sites for around $29.00 and up
Here is one such posting.
http://shopping.superpages.com/dremel~erci_42282~ty_au
It is quite a small lathe but should be fine for pens and bottle stoppers etc .
Will be interested to hear how much (or little) power it has when you get it up and running. And yes, please do post a picture of yours.
W.Y.