Hi John,
Using an electric motor, I have little experience of petrol engines.
Long shaft is definitely the recommended type, as shown in the brochures printed between 1970 and 1986:
http://www.seahawk17.org.uk/promotion/brochurereedcraft1970.htmhttp://www.seahawk17.org.uk/promotion/brochurereedcraft.htmhttp://www.seahawk17.org.uk/promotion/brochuremore.htmWhen it comes to swinging the engine forward to lift the prop clear of the water, a lifting bracket must be helpful, although to what extent that applies when using the Moore's style of main sheet (from centre of transom to boom) rather than the earlier "four-point" Reedcraft style:
http://www.seahawk17.org.uk/description/rigging.htm#mainsheetI am not clear - and I know you use the Moore's style of main sheet.
For the effect that the added size of modern four-stroke engine has, I will have to turn to others for advice... (though a photo I have of your existing engine suggests that the engine cowling is quite large and a modern four-stroke may be no bigger.)