A well fitting temporary crown (i.e., one that does not bother you) is not considered a chatzitzah unless it is very temporary. Halachic opinions vary somewhat as to the length of time that is too temporary.
We follow the opinion that it would be considered a chatzitzah if it is to be replaced within a week after immersion. Therefore, if the crown cannot be replaced prior to immersion, make sure to delay its replacement for at least one week after mikveh.