Not to worry, it is a pretty easy build... Hardest part is probably getting engine positioned on the rails... I mounted the Saito as far back (towards the firewall) as it would go and still was about a quarter inch to far out on the prop flange... No biggie though .. I just fit the cowl to the front of the engine and mounted it there... This left a little gap on the bottom of the cowl but that is good.. A good space is needed down there for ventalation... Worked out good... I mention this because you may run into the same thing with your OS 4 stroke...
IC