Unless I'm missing something, using different scales doesn't actually preclude utilitarian calculations.
There's no obvious natural conversion between fun and pain (hence the question), but any monotonic conversion function that we choose to adopt will give us well-defined values of X and N. We can construct a utilitarianism around any of these possibilities: if X or N are very large or infinite, that just means that we're approaching pure negative utilitarianism, defining utility in terms of minimizing suffering without taking fun into account. Deontological considerations need not apply.
Indeed, of the major utility functions that I'm aware of, this problem only seems to arise within pleasure/pain utilitarianism; as stated, negative utilitarianism doesn't admit to the existence of a finite exchange rate for torture (which seems rather shaky in light of preferences like those expressed in other comments), while preference utilitarianism's results are going to vary by individual and case but are always individually well-defined.
(This comment was originally buried more deeply, but on reflection it seems to have more to do with the OP than with the comment I was replying to.)
Most of the usual thought experiments that justify expected utilitarialism trade off fun for fun, or suffering for suffering. Here's a situation which mixes the two. You are offered to press a button that will select a random person (not you) and torture them for a month. In return the machine will make N people who are not suffering right now have X fun each. The fun will be of the positive variety, not saving any creatures from pain.
1) How large would X and N have to be for you to accept the offer?
2) If you say X or N must be very large, does this prove that you measure torture and fun using in effect different scales, and therefore are a deontologist rather than a utilitarian?