for the HT engines, the carb limits how far they can be pushed back towards the seat tube, and the spark plug limits the vertical space needs. Unless you have an atypical cruiser bike, basically it should fit.
I got a 65cc into this bike
http://bikepedia.com/QuickBike/BikeSpecs.aspx?Year=2006&Brand=Norco&Model=Wolverine&Type=bike
it has NO room to spare, so i guess it's as small as you can go. On the other hand, there is a guy on here fitting a lawnmower engine into a BMX via flexible jackshaft, so it all sort of depends on your desires/perseverance.
edit: Oh your planning on throwing them into road bikes, i see now. In that case, i would say it looks like they would fit, but some kind of mount will need to be fabricated i think, because that looks like a flattened aluminum downtube? If it is, drilling the frame is out as is the stock attachment methods. If you need to fab a mount it's difficult to say if the engine will fit or not. I would still say yes for the 52, but the 46's top tube looks like it slopes towards the seat. Your spark plug might have some issues there.
Sorry i cant measure mine, it's in the garage due to snow, and is slightly inaccesable.