Zardnaar
Legend
I didn't have any issues with the young black dragon in the Sanctuary of Elwyn the Ardent portion of B9 I think the characters were 5th level by that point and I know for sure they had hired on two other adventurers so there were 4 of them up against it.
What issues in conversion have you had? Did you use some method other than what I did, which was to just ignore everything in the original source except the age and color of the dragon and use the stat-block of the nearest possible found in the Monster Manual?
A few BECMI/AD&D adventures have older Dragons in them in in 5E you kind of have to use a young Dragon.
They had more age categories and the Dragons were weaker so in some wayts 3E for example you can get a Dragon vcloe to the required CR. There is a big jump in power between young and adult Dragons in 5E.