The ^ is the only broken operator i've found, so can be built by multiplying smaller numbers against themselves:

Code
//var -s %a.bf 2 ^ 52 , %correct.bf %a.bf * %a.bf , %wronggg.bf %a.bf ^ 2

* Set %a.bf to 4503599627370496
* Set %correct.bf to 20282409603651670423947251286016
* Set %wronggg.bf to 20282409603651670423947251286020