OK I did some digging and here is what I did.
I restored the previous database to a new schema name - forum999.
I then ran these three commands, with the missing thread's ID:
insert into ubbt7.ubbt7_topics
select * from forum999.ubbt7_topics where topic_id = 64605
insert into ubbt7.ubbt7_posts
select * from forum999.ubbt7_posts where topic_id = 64605
insert into ubbt7.ubbt7_topic_views
select * from forum999.ubbt7_topic_views where topic_id = 64605
It looks like it worked perfectly and it's back in business again.
Are there any other tables I should have filled in to go with these three, from the backup set?