Can someone please help me mod my paintball gun?

Hello
I'm building a paintball gun I plan to make it out of the stl files below.
I need help to stretch the magwell or magazine slot of the paintball gun.
I need expanded from the .75 x1.25" that is now out to
.9 x .1.8" So I can fit tippman magazines
I have a printer just no know how on the cad design.
I'd really appreciate the help if anyone has the time.
Thanks.

Here is stl file for it I need modded for the paintball gun I'm building.

https://www.thingiverse.com/thing:5908674/files

https://www.printables.com/model/425067-replica-tec-9-frame-scale-model