Do you have any chance to attend class before your exam? This would allow you to see how you arm holds up and then go from there.:
I did attend my class 2 days ago, and even did uke for my teacher, only attacking with the right hand. As far as the right hand, everything is OK. I keep my left a bit folded but otherwise the lesson went OK. My classmates accepted me attacking with the right hand only and being attacked likewise.
I will also attend tonight's lesson and see how it goes...