I did a similar conversion to my old beat up nextar after several minor mishaps, a decent crash, and some fuel soaking in a few areas,I stripped the fuse, glued some of the loose areas,recovered it and installed a OS 61 with a larger fuel tank.

I am about to switch out the 61 2stroke for a 91 4stroke its a bit cleaner, and quieter on the water, the 61's going into a FW 190.

some things you will need to do is relocate the landing gear,I made a new mount just forward of the 3rd former and it balanced out well with no added weight on either end,I used the Hangar-9 60 size landing gear and braced them with some alum tubing.

your also going to need to get a larger mount for the 61 the iso mount doesn't fit the larger engine, as for changing the thrust thats your choice I didn't.

I fit a set of goldberg superfloats to it, there a little large for the airframe but they float well and are a nice kit to build, with easy to follow instructions on construction and mounting.

as for changing wing incidence I havent heard of that being needed, I did seal the wing saddle with weather stripping to keep the water out, I haven't straitened out my wing yet though.