This book has several drawings taken from "mid-action" photographs. I believe Sullivan was one of the subjects being used to throw the punches. http://books.google.com/books?id=l1ECAAAAYAAJ&pg=RA1-PA1&dq=intitle:boxing&as_brr=1#PRA1-PA2,M1
One fighter who was compared to Sullivan is Terry McGovern. It might of course just be that it was the best comparison available. There have been comparisons in recent years that we can verify to be tenuous or inapropriate.