Alternative cranksets for a cross-country mountain bike larger than 48t
Wanting a bigger ring indicates you'd like a higher gear, not a lower one. Low gears are your climbing gears.

Part of the answer depends on which front derailleur you have. Front derailleurs have a profile to their outer cage that needs to match well with the profile of a given chainring size in order to perform well and not cause derailment. If you have a hybrid or trekking type triple FD whose spec sheet lists compatibility up to 48t, then pushing up to 50 might work okay, but it might also be pretty mediocre. If you want to really go nuts, some success can be had be grinding the shape different, but it's easy for this to go south.

I don't think that 50t 104mm BCD chainrings for derailleur use exist. Narrow-wides in 50 and 52 do exist if you wanted to 1x it. There are numerous questions here about how to determine if you want to do that.

If you wanted to spend some money, you could easily do what you want here by getting a road shifter, front derailleur, and crank.

Many bikes with the sort of gearing you've got will encounter clearance problems with the chainrings if you go any bigger.