Hello,

I have a Dave Brown engine mount which I drilled to fit a Saito 91
Unfortunately I drilled it too close to the firewall and need to
bring the engine out about 1/2 inch or so.

Would drilling over new holes and using it with the old holes
have any structural problems ?

The mount is quite beefy.

Lennyk

Extra holes pose no problems (except, may be, aesthetic ones). Dave Brown engine mount (the black plastic one, right?) is by far one of the beefiest links in the load path. During a crash, everything else (engine bearins and crankcase, firewall and the fuse) will be destroyed but the engine mount will survive even with your extra holes.

Yeah, its a largish black plastic, 90S model

If your moving engine back I would, but moving engine forward leaves 2 holes to the rear of the engine. I'm very familiar with these mounts, and yes they are very durable and stout, but moving engine as you are leaves a weak spot right at those rear holes that were to be used for the first location.
See,,,if you were moving it back, the engine beams itself would act as a structure, but moving forward is definately not advisable because of the void it leaves in the rear holes. I've seen them break from doing exactly that.
Your choice though, I would save mount for future plane, and spend the $6 or so for new one. I've personally bought more then one mount because I wasn't satisfied with initial placement of engine. Cheap insurance for you at the price of a mount. Good luck