“in a minute” can be “mewn munud”, “yn y munud” or “mhen munud” - all are equally correct, just a case of personal preference.
The “now in a minute” is a particularly Wenglish phrase and doesn’t have an ‘official’ translation, but if you want to add a ‘now’ to ‘in a minute’, just add nawr or rwan (again, personal choice - nawr is more common in the S, rwan in the N) to the beginning of whichever version of ‘in a minute’ you prefer!