It is possible that the shift mechanism can be removed from the integrated shift/bnrake lever and a new shifter can be removed from its mount and bolted in place where the old one came off. This was possible for a abunch of shimano shifters - very likely if the shifter can be removed with one allen head bolt from the top. If the shifter has multiple bolts holding it to the integrated shift/brake lever mount, it is probably not possible.
Another option is to remove the shift mechanism and hacksaw off the part of the mount that does not affect the brake lever, then get a separate 7 speed shift lever to mount beside it. The one possible snag with this strategy is that the exisiting handlebar clamp might be too wide and interfere with some new shift levers.
The best 7 speed shifters I know of are:
https://www.amazon.ca/SHIMANO-Altus-.../dp/B0833BY837 but other options are available.
Also, if your bike has V-brakes (linear pull) then just about any modern brake lever will work if you decide you don't want the hassle of combined shift/brake levers.