I just spent a few days getting my ST GS40 powered 4*40 ready for this summers club races. It is 6 yrs old and never been dingied.
My big change was mounting the ST 40 on the bottom of the engine mount. I couldn't access behind the firewall to change the mount location so I put the engine on the bottom. One allowable modification to the 4*40 is dropping the engine 1/4" below the specified mount location to avoid a drop in rpm resulting from a high G turn. At about 15,000 rpm my ST 40 would drop rpm very slighty when pulling out of a turn especially on the last 3 or 4 laps of a 10 lap heat. The only changes I would recommend would be to make sure the stringers behind the canopy are hard balsa and increase the span of the hole in the balsa piece under the engine. I don't see a problem with the landing gear other than weight however we are not allowed to bore holes for weight reduction and make sure your aileron linkage is tight - don't use metal fittings - way to sloppy.