I have a similar issue to mic (broken knee joint of a figure, have been able to at least dissemble my patient) so this info is useful, is this the type of glue you mean?
http://www.ebay.com.au/itm/331795841448
Would this be the best fix for these types of breaks, ignoring of course a complete knee replacement from a donor bot?
mic: is it possible to dissemble the whole leg somehow?
I think that's your best bet as that way you can repair the knee properly and slot it back in after it's dried so you wont have an immovable knee