The lightest .15 engine I have is the Norvel .15, it weights 5.3 oz without a muffler. A typical .15 with muffler will be around 7oz.

The recommended weight of 18oz suggest a .061 or a Norvel .074 engine to me. The wing profile of these are not suited for high speed, and neither is the stock decalage (that can be changed though). Why not just convert a dedicated high speed plane instead? There are plenty of strong and slim airplanes to choose from...