Hello and welcome to FT

It entirely depends on whether Cathay Dragon (KA) will tag your bags to SIN - most likely this will be refused when you are on separate tickets. Moderator please move to the CX/KA thread as the OP will get better advice there.

If bags are not checked through to SIN, you need to clear immigration at KUL, pick-up the bags from the carrousels and proceed to check-in counters before the check-in deadline. Which airline to SIN? If MH/SQ check-in is easy and I would say 2-3 hours between the flights should do it and you will most likely be fine. If you are flying AirAsia or other LCC to SIN you need to change to different terminal at KUL and need more time.

Anyway, you should consider buying a ticket HKG- SIN directly - not worth the hassle and extra time required to fly via KUL to save a few $ IMO.