As the title implies, can you craft a 9th level spell scroll to cast a 1st level spell, making the spell stronger?

Iow, can you make an upcasted scroll.

imma say no.

Based on the standard rules, I would also say "no" to your question about crafting an upcasted scroll. However, it's a question that might be better suited for your specific DM. They might be willing to allow you to craft a more powerful version of a lower level spell if you meet certain criteria.

For example, while unsure why somebody would want to create a 9th-level spell scroll for a 1st-level spell, I would be willing (as a DM) to hear a player out if they'd be willing to commit the time and gold required. So, referencing Xanathar's Guide, it would cost 250,000 gold pieces and 48 work weeks to make that 9th-level magic missile spell scroll.

RAW it doesn’t say, but there’s no precedent for it this edition. Keeping all 1st-level spells as 1st level spell scrolls with fixed save DCs keeps things easier for people to remember and reduces bookkeeping. This way nobody ends up with three scrolls of burning hands all at different levels with different save DCs. See what I mean?

It used to be something one could do in older editions however. As a DM I would allow it if it was something a player wanted to do. But it would be up to the player to keep track of with their own bookkeeping since, as a DM I have enough to keep track of for myself.

I mean, the save/attack bonus is still fixed if you use a higher level spell scroll. I dunno, I'd go for it.

I don't consider game breaking i would allow it

I don't think I'd necessarily have them as random loot, but I also don't see a reason why the RAW makes writing an upcast scroll impossible, and pays the appropriate costs.  

Of course, I could see powerful NPCs perhaps using some of their treasure resources to produce similar scrolls so the magic fight just got a lot hotter.  If NPCs and high level party members have a lot of downtime.

Upcasting is generally worse than using a spell native to that level. For example, Fireball is natively 8d6 over a 20ft radius sphere. Upcast to 9th level, it hits with 14d6 over a 20ft radius sphere. Compare that to the natively 9th level spell Meteor Swarm, which hits with 40d6 over four 40ft spheres (so almost treble the damage over sixteen times the area, assuming we're looking at the ground footprints of the spells, which is usually what matters - looking at volume makes the discrepancy even greater). Upcasting is generally what you settle for rather than what you want to do.

Therefore, if they're paying the price for the level it will be cast at, I have no issue letting them do it. I haven't checked the wording on RAW, but I don't have any qualms about it. If they want to waste their resources on upcasting a scroll, I won't stop them.

Agreed with Linklite - upcast spells are usually weaker than spells of that level, so I would allow this as long as the higher spell level is used to set the crafting costs or purchase price.

This also means these scrolls would be suboptimal for scribing purposes (as the wizard will get the same spell they would have gotten if they had gone for a cheaper, minimum-level scroll.)

The upshot to this is that you can find  or make some useful upcast scrolls, such as a Fly scroll that can support 4 people.

I mean, it’s mostly straight damage spells that suffer from upcasting; I don’t know that there’s any other spells that are better for paralyzing targets than Hold Person/Monster, and upcasting Bestow Curse not only increases the duration, but can also remove the concentration requirement.

Healing scrolls instead of potions is one use.  Bigger and easier top up than a potion and don't waste a valuable spell slot when you don't need to.  Magic Missile for when it seems like and AC 30 or something inane.  I can picture a few, niche uses for upcasting to a scroll.  I'd allow it, as stated above, with increased cost to match the upcast level.