Hey there...

I've got a few of the bigger Spooks (I believe the original, not the Super) with two treble hooks and they need to be changed.

First of all, which hooks do you recommend and what's the easiest way to swap the hooks out?

What hooks would I recommend? Daiichi Death Traps (sticky sharp)

Best way to change them? Very carefully  

Gamma round cut off old hooks install small split rings gives a nice action to spook JMHO

Also is there an easy way to determine the hook size so I get the right size?

Also is there an easy way to determine the hook size so I get the right size?

The easiest way is to take one with you.  

Yeah, just take one with you.  Also, be absolutely sure to get the same size that is on it.  My friend swapped his out for a smaller size and he couldn't hook any fish that blew up on his bait.

The split rings are a good and easy modification too.