Can you sell a house to cover cost of "street repairs"?
You can sell houses and hotels at any time, for any reason.
From the rules:

Houses and hotels may be sold back to the Bank at any time for one-half the price
paid for them.

The amount of money you have to pay is based on the houses/hotels you had at the time you drew the card. Selling houses/hotels at this point would not reduce the amount you have to pay.
While this is not addressed directly in the rulebook, multiple sources confirm this in a FAQ.
http://richard_wilding.tripod.com/faq.htm

When assessed for street repairs, pay for all houses and hotels you own on the entire board at that time.