Go for an FAA PPL. You will be credited with the hours with the ICAO compliant instructors who trained you for the NPPL SSEA. The FBO will sort the TSA clearance for you. They are legally obliged to screen applicants and will not apply to the TSA for clearance unless they are satisfied all is in order. It is not as if they are teaching you to fly and "the FAA have confirmed 14 CFR 61.3 permits the holder of a UK-issued sub-ICAO licence, such as the NPPL or LAPL, to operate a US (N) registered aircraft within the UK."
Your 3-axis microlight hours will count for experience too.