The biggest issue that I've heard of with the Gilberts is that the heads are rather fragile - the .07's have the reputation of blowing the glow head on the first run or sometimes even with just a battery hookup, so start with a glow driver turned all the way down.
I made a head for one a long time ago that took a regular Standard Short plug but I've heard also that the stock heads are a very soft aluminum alloy that is difficult to drill and tap.

These engines are still available from somewhere, as are parts and spare heads.