I've seen Honda 4 strokes in nearly every pre '75 frame imaginable and nearly all of them have been less than successful, each for its own reasons. I personally have used Montesa King Scorpion frame...(too heavy and the bottom frame rails need to be lowered to get the sprocket as close to swingarm centre as possible) and CZ (Same work as the Monty although much lighter). As good handling as the Maico is, it's not the ideal candidate either although the engine does fit a lot neater than most. The major problem being exhaust clearance with the twin downtubes but it's not insurmountable.
My pick is to use a circa 71-74 MK series Husqvarna frame. I've seen a few of these conversions and the actual swap seems to require less surgery than most of the others. One of the quirks of Husky handling is their slow turning ability, needing a bit of body english to place some extra weight over the front to rail the berms. It's not a fault, it just requires a style adjustment from what you usually do. However, it's been found that the fitting the much heavier Honda 4t engine turns the Husky into a cornering weapon, they turn much quicker than a stocker...one of those little bonus's that often pop up from an engine swap that you least expect.
Having said that, the most successful four stroke Hondas from the golden era of pre '75 nearly all featured stock frames. It's surprising how much weight can be chopped out of a stock XL frame and once fitted with decent forks (Ceriani, Betor, CZ), lightweight hubs and plastic or alloy tank and light plastic mudguards and side panels the XL stocker isn't as mundane as many think. Back in the 90's Steve Greentree raced a beautiful pair of XLs that were practically unbeatable, even outside of the 4 stroke class. These bikes rocked and inspired a lot of other builders to use Steves subtle tricks to get the old trailbike working well.
