A seventish four stroke would likely be just fine- but What is the weight of all the mods, no one here can answer that question for you. A stock Senior Kaydet can handle a a two stroke from anything from a modern .35 to perhaps a good .75 with out any mods or problems and with four stroke and anything from a fiftiesh to a good .80 or so.

All those I mentioned without hardly any mods. Now down to brass tacs and opinions, only just my opinion that is:

While my advice is always find a mentor no exceptions however If you Are going to go it alone : First reread mike 109's post above and list for doing so, its good advice. I am only going to address the engine choice and you asked but restricted the engine choice to four strokes.

Anyway here goes:

Without any other info about these mods especially weights and how much the gentleman has increased drag (You can beleve things like struts, fat fuselages or whatever does have major effects) I would stick with his engine of choice the 91 fourstroke.
Now just speculating on the effects of the mods my engine of choice would be an OS 95AX .

I have flown and developed a variety of mods for the Senior, actually an altime favorite airplane for many years. Some very outlandish and I do currently fly Seniors with 87 wings.

I do realize barking up a two stroke engine in these forums is normally futile but like I said I am going to state that opinon.


Enjoy

John