Mario Bros is a great game and an even better 2-Player game. Yes, I'd say the game has increased in popularity lately.
The dedicated widebody is worth more so since yours is a conversion, it's worth is a little less.
From only looking at your picture and assuming everything works 100%, I'd estimate $750ish.
If you could see that machine to the finish line with side-art, orange speaker grill, high score save kit and (maybe real or reproduction Nintendo buttons and true-flat t-molding) (hard to tell from your pictures for sure)...I'd say you might convince someone to pay closer to $900.
It's going to depend heavily on your location. Denver is red-hot right now so it would sell easily. Not sure in your location.
Also going to depend on if you list here on KLOV or not.
Others could certainly disagree with my assessment but anything made by Nintendo is popular right now.