I highly doubt a quarter inch will make any difference. of course it will show up in the location of your CG, but a 1/4 one way or the other of spec'd location, is well within tolerable limits.
if you want to move the engine ahead, you can always epoxy a 1/4 plywood spacer to the firewall and drill new holes. that will also give a weak firewall some strength. with no cowl on the nose it won't look wrong for the front of the engine and/or spinner sticking out that extra distance.
I fully understand the importance of correct balance and i'm not advocating being sloppy about principles, but sometimes we get too excited about small issues that won't make any noticeable difference in how the airplane flies, just for the fact that it isn't perfect. if you run into someone that claims he can tell the difference in a 1/4 inch of CG location, you just met a big BS'er.
use your head before diving into any perminant change,.....1/4 inch on a big stick,....no problem,....1/4 inch on an .020 size plane might be a real problem.....it's all a compromise and all relative to size of plane. when that plane is 50 yards away, are you going to be able to tell that it is flying a half inch tail low ?.