Its been quite a while since classic for me. But I do recall there are several files impacted by deleting a forum. I did the same thing as you trying to perform some housekeeping.
I remember there was an issue with at least the 5 series when deleting a forum it was suggested to never delete a forum but to disable it. I can't recall what I did to repair it. Possibly try a search posted by charles capps. I believe that once you delete a forum that forum number will never be reused, If you were to add a forum it would be Forum 34 and rebuilding the stats would just skip 30,31,32 and 33 and perform a rebuild on 34.