LimeSDR external clock GPSDO

What external clock can I connect to LimeSDR v1.4 for stable work with Osmocom GSM stack ?
ClockTamer is no longer available, and they are very expensive,
perhaps there is some replacement that will achieve acceptable accuracy for GSM (0.5 ppm)?
Do I need to make an HW modification to connect an gpsdo to LimeSDR v1.4?