Is the tunic knitted on the diagonal (that would be difficult I think) or is the stitch pattern on the diagonal? Maybe you could find something similar and add the stitch pattern? I'm just thinking about what I'd do in a similar situation. With all the patterns on ravelry and other sites I think finding something close enough to work with could be a viable option. From your description it sounds like a pullover, is it?
I should never overestimate my basic arithmetic skills.