Package length:
Option one: Use the first byte as package lenght
Option two: Set PKTCTRL0.LENGTH_CONFIG=0x0 and set the package length with the PKTLEN register.
Sync:
Change SYNC1 and SYNC0 to your wanted SYNC word, use MDMCFG2.SYNC_MODE to set 16 or 32 bit sync.
Preamble:
MDMCFG1.NUM_PREAMBLE to set the number of bytes