If you look at the weight of a gas engine - say 40-50 cc- you must include all accessories which make it a viable quiet setup-
So if the basic engine weighs 2.5 lb -or 3 lbs - -that weight is only part of the "power unit".
The total is more like 3.5-4 lbs .
On a percentage scale - a bare engine weight savings of 8 ounces (optomistic savings) figures out on a 3.5 lb package as 8/56-or a 14% savings on weight of the "power unit".

A 32 oz 40-50 cc gasoline engine may be a possibility -
In any case - lets see what actually transpires first---
As an example -
When I tried making 11 lb 1280 sq in model - with a 40 and a pipe - I was wondering how to do it- and ended up under that weight!
One never knows how light stuff can be until it is really given an effort.