Having done several conversions of BECMI adventures, I've found that most Basic adventures are suitable for level 2-4 characters in 5E. Level 1 characters are generally too fragile, and those adventures generally assumed ~8 PCs (with deaths common). If you want to run them with level 1 characters, I'd suggest breaking up the adventure, making the start easier and allowing a point to level up the characters (either as a milestone or using xp). Once the characters are level 2, you can start bring the difficulty back up.

Some other suggestions:
* Unless I really feel like making a conversion, I often substitute for existing monsters where possible.
* Cut non-magical treasure by at least half, probably closer to 10%
* Remove all non-necessary magic items, then re-add level appropriate ones as needed (should still be a lot less)
* Often the idea of the adventure is the best part, then simply fill in the adventure with level appropriate challenges.

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