I would thin out the airfoil to some 15% root 13% at tip to help with penetration. It will still have plenty of lift.
To make a light fuselage you might want to consider laminating two layers of 1/32 balsa over a foam plug to make the fuselage shells. Maybe cut the number of ribs in half.

So if I just go to the copier and scale down a Thundergazer to 1/2a size will it fly ok or does it need a bigger tail or something?

Thanks,
Motorman

Having flown a gazillion 1/2 A's and also free flight when I was younger I don't think that weight is that much of a factor (within reason).  If you have ever seen a TD .049/051 lift a 300sq" free fight (that weighs probably twice what you are about to build) straight up accelerating as it goes you know what I mean.  IMHO line tension is the #1 problem in 1/2a.  Too light and it doesn't stay out on the lines or you have to use lines too short to do a respectable pattern.  I wouldn't change the design at all.  I had plenty of planes with thick airfoils and they flew just fine although only a few had flaps.  If I did anything, I would build it so that I could move the engine forward or back to get the CG.  Every full body 1/2a I have ever built ended up tail heavy.

Good Luck - Ken