The ender 3 is already a waaaaaay better printer than mine in how it’s constructed. The only things i would probably change is to stick a direct drive extruder on and a single Z motor that mechanically couples both sides.
Probably the most effective upgrade is the single motor Z axis mod. The leadscrews used to bind constantly as the motors got out of sync very easily. I’ve heavily modified the original single motor upgrade to allow a cable chain to be run as well as an XT30 plus DB25 connector to disconnect the head.
If you set up the klipper firmware you can get silly speeds out of the printer without ruined prints. I’ve been able to print at more than double the speed that i could with marlin with all these mods.
PETG is a very finicky material to get the settings right with. If the print has a matte sheen, temperatures are too low, if it’s too high you’ll get stringing all over the place. It also picks up moisture at a rapid pace but you can put the spool in a food dehydrator for a few hours at 65 - 70C to remove the water. If you leave a spool out for a few days it’ll absorb enough moisture to ooze and ruin prints.
Temperature and retraction are the most annoying settings to get right with this plastic.
From what i can find, the mk1 speakers have 3 screws rather than the 4 of the mk2.5 as well as the area being differently shaped.
I’d like to design an adapter for the MK1 but that would require access to a MK1 door and door card as it’s all designed around the contours in the door.
Even some rotten or dented door would work, so long as the speaker area is intact.
I have some of my printer mods on thingiverse if you want to take a closer look.