Custom Criical settings(Shaped 5e sheet)

I am using 5e shaped sheet and was wondering if there was a way to invert the nats of a certain item? Ie a longswordnthat crits on nat 1 and fails on a nnat 20

This is more likely a general Roll20 dice rolling expression problem, rather than something specific to the Shaped sheet. However, any answer you come up with will probably need "dice modifiers" checked in the Actions box of the settings panel of the sheet. 

Anything supported&nbsp; <a href="https://wiki.roll20.net/Dice_Reference#Roll_Modifiers" rel="nofollow">https://wiki.roll20.net/Dice_Reference#Roll_Modifiers</a> &nbsp;is supported on Shaped via dice modifiers.

I will look into it thanks for the input

One way to somewhat accomplish might be setting as a normal weapon attack, but leave the actual damage field empty and fill in the formula you want to use for damage dice in the "Content" field as an inline roll, something like '[[1d8+@{strength_mod}]] $type damage' (adjusting the damage die, mod variable, and damage type as you wish of course) which would at least prevent the damage dice from being doubled on a 20, and remembering to manually retrigger it on a 1