I am considering the same change, for the dame reason, intending to relocate the shifters to the handlebars on my flat- bar AM14.
The rear mechanism is a Shimano SC105 short cage. I believe the cable pull values for most 7 speed changers are fairly universal, but I am not ” Sheldon Brown”
There are plenty of 7 speed shifters at modest cost available on the market, 9/10 speed block technology is so prevalent and our 7 speeds is (wrongly) associated with utility and budget cycles.
I have ordered a Shimano SiS changer from Decathlon, it is used on one of their B-fold folding bicycles and the quality and ease of use proved tp be satisfactory well on a test ride of the folder.
The shifter has a lever to change up the block (gears 1 towards 7) and a push button to increment down the block, the price? £10.