Yep, "Old" minibikes dot com. Most of us are here for collecting and restoring foremost, and riding is secondary. At 6' 1" and 235, I ride my small bikes, and my larger bikes with no problem.
If I were real concerned about "ridability" I'd go ahead and get a full size motorcycle.
If I were real smart, I'd buy up all the local Chinese bikes, fix them up and flip them, since no one in their right mind wants to spend $800 and more on a vintage minibike in pristine condition. (Except us knuckleheads on Oldminibikes.com) Price before value.
Cliff, do you do those mods to facilitate the larger engine, or to beef up the plate/frame? Have you thought about doing a jackshaft/swing arm mount since you're about there anyway? Looks very clean what you're doing.