Hi there. I've got this kit in my stash so I dug it out to have a look. I reckon my drill bit is also broken as there's only about 3 mm of drill sticking out from the plastic sleeve.
I've checked all the instruction sheets and bits of paper, but the actual size of the drill isn't mentioned anywhere. There is an order form you could use to order replacement drills at $5 for a pack of two but the address given is the original address for the AMT company. The address is: Drill Bit Offer, c/o Graphic Resources, Inc., PO Box 2786, Cedar Rapids, IA 52406-2786, although I doubt whether they're still there!
Just looking at my drill, I'd guess the size is just under 0.5 mm diameter. I've got a pin drill and a set of drill bits like this one:
20PC Titanium Micro Twist Drill HSS Suit
Airfix Models | eBay
As and when - if - I start this, I'll be starting with my smallest drill and working up 'til I find a suitable size. I would not have thought that the size of the hole for the optic cable was too critical. You're going to have to fix them in place with a spot of glue ( I'd push the cable right through the hole and apply a drop of glue to the back of the hole, then pull the cable back gently until the end is flush with the surface. That way, you're less likely to either block the hole or cover the end of the cable with glue). So as long as they don't 'rattle around' in the holes, anything close will do.
Can't tell you anything about the build quality, but I hope this helps.
Gern