Hi Dave,
Pic's on the Aeromarine site and the Shipwrightshop site seem to be pretty sparse, and there don't seem to be any of the "business side" (underneath), but it looks like it's probably a circuit racing type hull. If so, something to bear in mind. Circuit racing hulls for todays .67 size engines (the US still have a .67 monohull circuit racing class), tend to be at least 42". Their boats are running up into the mid 60s (MPH) - in full race trim (on the "wet" side), so I think a 36" circuit racing hull with a CMB .67, would probably be undriveable - especially on rough water. The Prestwich Orion, which is his current offering for .45 sized engines (albeit geared .45), is about 4' long !
Like Danny, I think a .21 would work pretty well - as well as giving a large choice of engines, many of which are both good and affordable - like those Danny mentioned (I think the Force .21s are even available in a Marine version). The smaller engines - ie .12 and .21 - those sizes used in 1/8 and 1/10 scale cars, are the biggest market, so this is the area where most of the development takes place. Most current .21s have the same mounting bolt pattern, and a high percentage of .21s use a 4 bolt head pattern, and the bolt pattern is usually the same, so a marinised head for one engine will fit many others. Four bolt marinised heads can also be bought quite readily. I'll post an eBay link if you are interested ?
If you should decide to go for a .21, a gearbox would give more flexibility. For one thing, the engine pulley / gears will normally fit straight onto the SG cranks used in car engines - whereas a flywheel for direct drive would be a little more difficult. Gearboxes are also particularly useful where you are driving a biggish boat - not least by helping you to run a bigger prop. On the other hand, a gearbox is obviously an additional cost, and maintenance item.
As in your post though, your older .61 engine would probably also be quite decent - just got to work out the plumbing.
Ian