Hi,
Item journal is best for positive/negative adjustment of products.
You can try below example according to your scenario:
Negative adjustment: For example, lost 50 items, do 50 items negative adjustment and suppose it's generated item ledger entry No.=242
Positive adjustment: Found 20 items, do 20 items positive adjustment with "applies-to entry"=242 and if you want to link item with purchase(vendor) then fill "source type"="vendor" and "source no."="vendor no.". Additionally, you can use external document number as purchase invoice number.
For remaining 30 lost items, you can do same positive adjustment if found again.
I hope this will work
Thanks,
Sagar Dangar