Repowering E27 outboard

I'm looking at buying a 1972 Ericson 27 with a dead outboard. This is apparently a model that came with a factory-installed outboard, but the current dead outboard is not the original one.

What type of outboard should I get to replace it with?

Most importantly, how much HP do I need? But also, are there any sizing issues I need to be aware of? I know that I probably need a long shaft motor, but how do I make sure the new outboard will fit in the existing transom gap?

I've attached some pictures of the current dead outboard.

I have an E 27 with an outboard bracket and use an evinrude 9.9 hp outboard. You will want either a saildrive model or at least a longshaft. Nice options are electric start and alternator equipped. I have had three saildrives on various boats over the years and love them. The 9.9 pushes the 27 along pretty well.

Some information

With a little site searching, I found some relevant threads on this.

http://www.ericsonyachts.org/infoexchange/showthread.php?t=1514&referrerid=28

http://www.ericsonyachts.org/infoexchange/showthread.php?t=2484&referrerid=28