Having done 6 ocean crossings in light twins and 2 in a cherokee, Here are some suggestions. If you're flying a plane with a couple of thousand hours on the airframe and maybe 500-1000 on the engine, the probability of a failure during the 10+ hours between California and Hawaii is very small. Nevertheless, don't skimp on survival gear. Don't even consider going with a freshly overhauled or new engine with 20 hours on it. Don't go with an airplane that has just flown off its restrictions. You'll need/want an HF radio. Its true that routine communications can be relayed via vhf from airliners, but we're talking emergencies. You'll need an HF antenna. Thats a bit tough, but not impossible on a glass plane. The airframe is a poor counterpoise. All transoceanic flight (above 5000 ft) is IFR even if you're in VMC. That means currency and competency. You'll need lots of planning. The north atlantic is simple compared to what you're attempting in terms of visas and permits. A north atlantic round trip, in a 180 HP cherokee, from VA to Glasgow cost about 3K in the late 80s. I'd guess its twice that or more today.
Flying the ocean is very easy, planning and preparation is significant, but very enjoyable. Its very doable, a bit pricey and most fun. Think about a prep flight to Greenland. Rent a kayack and spend a couple of days on iceberg laden fjords. You'll love it.
Have fun,
Marc LongEz 770EZ