I file single and defaulted on $1900 401K loan this year - serious car accident and out of work….can I claim it was d/t being unemployed or med expor better to repay it?

The amount that counts as a withdrawal will always be subject to regular income tax.  There's no way around that, the contributions were tax-free, so withdrawals are always taxed.

There is an additional 10% penalty if you are under age 59-1/2. There are also several exceptions,  they are listed here (note they are different for 401ks and IRAs).  https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-exceptions-to-tax-on-early-distributions

Notably:

• If you left service with the company and are age 55 or older
• If you used the money to pay for medical insurance while unemployed
• If you had medical expenses that were more than 7.5% of your adjusted gross income
• Up to $1000 for an "emergency personal expense." (New in 2024)

Being unemployed is not by itself, an exception to the penalty.  But you can claim $1000 as an emergency personal expense and just pay the penalty on the other $900.  You might also have medical insurance or medical expenses that would count.