I'm not sure I understand why you think that you can't use a rotavator, I was using my dad's Howard Gem
like this but with a different engine when I was eight... the good ones have adjustable handlebars to suit the operator... certainly all Howards had them cos even my dinky little Bulldog (the smallest proper rotavator they did) has height adjustable bars....
You can fgrequently pick up rotavators on ebay for sensible money, I paid less than 80 quid for my Bulldog (and it works properly) though don't buy a "non-runner, easy to fix" one.... if it was that easy they would have fixed it themselves..... My other rotavators (a Howard 400) needs mer to strip the engine and gearbox down this winter cos of that, whatever the problem is it won't be that easy :D.... that siad it only cost me 60 quid so if aI have to machine up a conversion plate and fit the spare Honda engine I have lying around then it won't be too bad.....
chrisc