How to create a Stat-boosting magic item?

One of my players has a character (imported from a face-to-face AD&D campaign) who has several Ioun Stones, a couple of which provide stat boosts. I'd like to be able to recreate them in such a way that if the item is lost, or if another is gained, its effect is automatically reflected on the character sheet. We're using the default D&D5e sheet. How can I do this? Can it be done at all?

I don't know that it can be automatic.  I would personally make a notes handout for yourself and note that the character has those items and that they are providing the bonus they do on said character's sheet.  Then make those adjustments to the sheet.  Then trust yourself to make the negative adjustments if and when it become necessary.  Oh yes, you could also make a handout for the item, viewable only to the PC (always viewable by GM).  Place it right with their character sheet in the game journal.  You'll probably be reminded of it there more so than on a note to yourself unless you revisit the note often.

Assuming I'm reading correctly, this is built into the equipment are of the sheet. Each equipment item has a field called "mods". This field can adjust attributes and ability scores. From the wiki: MODS:  This is a text field where you would take note if an item (likely magical) would modify your stats in some fashion such as a character's Attributes, Saves, Skills, AC, To Hit, Damage, Spell Attacks, and Spell DC. If a stat is modified here, its attribute value will become highlighted in blue and adjusted by the supplied modifier listed here. An asterisk is appended to the right of the value to also signify that it has been modified by an item. While your mouse is hovering over the attribute, it will revert back to its unmodified total so you can refer back to its pre-moddified value. The format for a stat modification is  stat name:n  where n is the stat bonus (ex.  Strength:21 )

keithcurtis said: Assuming I'm reading correctly, this is built into the equipment are of the sheet. Each equipment item has a field called "mods". This field can adjust attributes and ability scores. Edit: Aha, found it. Thanks :) It looks like it will only accept an absolute value, not a plus or minus value. I'm not seeing any such field, or maybe I'm looking in the wrong place, or using a different sheet. Is this the equipment area you're talking about?

Yeah, it's not a very intelligent modifier. It modifies some attributes (ex. AC), but replaces others (ex. Ability scores). The Shaped Sheet handles this stuff much better. The D&D 5th Edition by Roll20 sheet doesn't handle Ioun Stones at all. One very important thing to note: If you are using charactermancer, be sure to unequip any item that adjusts an ability score. Charactermancer doesn't recognize them as modifiers and will overwrite the ability score with the adjusted one. Unequip -> Level Up -> Re-equip.