I'm not sure what kind of cranks the stock mundo has. If the rings are removable, you could get a 48t chainring and maybe a 50t if you're lucky, assuming 4 bolt 104mm bcd (bolt circle diameter).
The more pricey but easy option would be to get a road triple crank and chainring setup, such as a sora or tiagra one. (it's 9sp but that won't matter, worst case scenario, you might need to upgrade the chain too, I don't know what the stock yuba has and can't tell from pics on their site, 8sp chain will work great as upgrade)
I google it and found one for <90, but you can probably do a bit better looking around
Shimano 2011 Sora 9-Speed Triple Road Bicycle Crank Set - FC-3403 : Browse All Products
(edit: just noticed this one doesn't include the necessary hollowtech II bottom bracket, look around as I think you can find a road triple for around this price that does include bb.)
Sugino makes a nice forged square taper crankset that *might* work with your current bb, but this depends on its length and once again, can't tell this spec from the yuba site (though no manufacturer gives this detail)
here's the sugino:
Sugino XD600 26/36/46 Triple Crankset in Tree Fort Bikes Cranks (cat119)
and you might need to buy a new bb too, but you won't know until you measure the old one (spindle length[~100-125mm range]) and compare it to what the sugino requires.
Once you have 5 bolt 110mm bcd you have a lot more chainring options. I have a cheapo steel 50t 5 bolt ring I got for nearly free from a shop that does used bikes.
The complication is that your front derailleur should roughly match the curvature of the big ring for good shifting. If you go substantially bigger than what you have now, your derailleur might be sub optimal, but you can probably live with that. You just have slide it up the seat-tube until the derailleur cage clears the big ring.
I'm not sure about how small of a gear you can have on the rear wheel, as this is an unusual wheel, but you do have the option of putting a regular cassette w/ 3/8" to 14mm axle adaptors and then you can go up to 10sp and down to 11t, though changing the number of gears would require new chain, shifters, etc. Replacing your cranks would be the cheap place to start.
Regarding Fox's comment, yes high bottoming will be more problematic in off-road situations, so this could be applicable, depending on where you take the bike.