2 things: Why not learn a NEW framework? A monthlong jam give you plenty of time to learn new ways of doing things, and is the main reason I personally even do jams, and two, why even post this? you CAN make platformers in Scratch

Try construct. My friend who uses scratch moved to it with my help and he likes it.

I don't want to slow down your enthusiasm, and what i'm about to say is based on my guess of your experience level, which i might have wrong... so take this with a grain of salt.
But if you're at the stage where you still need to learn a game engine, i would personally focus on small, but complete projects, instead of a large and complex project like a metroidvania. That's more likely to end successfully.
You can still do whatever you want anyways, if that's more fun for you... just don't feel disappointed if the deadline hits too soon... you will probably learn a lot anyways

I use scratch also and am also looking at switching to the free version of construct but might give scratch one more try

You can make nice games in Scratch if you use a bit of ingenuity to overcome its limitations. My daughter participated in two game jams with Scratch at Cambridge Center for Computing History and won one of them so it's very much doable.

For example Scratch is terrible with scrolling so split your map into rooms that you can fit on one screen (like Celeste).