I like your idea Ian, achieving it is the big question, as you have found out!!
What I envisage and would use, would include such examples as this http://www.theultralightplace.com/sparkplugs.htm
and this http://www.rotax-owner.com/
and this http://www.ultralightnews.ca/
and for something different this http://wind.willyweather.com.au/wa/wheatbelt/gabbin.html
and something like the articles (as an example) by Mike Stratman from California Power Systems called "The Care and Feeding of the Rotax Motor". I suspect this is just a small example of what you would want in your ultimate dream.
My suggestions are largely about the Rotax engine, by necessity this is the knowledge I have been chasing, now multiply that by all the major areas in recreational aviation that people will have a need for knowledge, then you will have a web based resource!!
Can you do it?
What ever happens Ian, my favourite section will be the forums, because of the interaction between all players.
Pud