You've got a couple of difficulties to overcome:
Just as Maros-Vásárhely (aka Neumarkt) is now Târgu Mureş, so your "Szent-Márton" is now something else (even if similar). However, there were at least 12 Szent-Mártons in Transylvania (3 in the same county as Maros-Vásárhely!) - did your relative mean the closest to Maros-Vásárhely or the biggest or best known? http://www.bogardi.com/gen/
has good searchable gazetteers. (Or use Wikipedia, WeRelate, FamilySearch,...) You also could google the on-line Romanian phone books and search them for where there are Csonkas now and then match the current locations with the old placenames.