Which motor? and what to do?

Gday to all, pretty new to the mower scene and currently building a Murray pan frame into an all terrain off road mower, it came with a 12 hp flat head briggs in it and I have a chance to purchase an 18hp opposed twin briggs also, thing is I have fitted a 12.5 oppy to it an physically it was too big to fit all what I intend down the track, alternator  for one as the top pulley conversion will not fit under bonnet etc etc.
So my question is what would peoples opinions be on these two options, would you go to the trouble of fitting the opposed twin and maybe bonnet scoop etc to fit and more modding, or would I get a good performer out of building the 12 hp flat head, it needs rings at the least and a head gasket.
I really wanted a twin and not scared to mod either, but would I be gaining much or should I just build the single stronger, is weight gain an issue, I have a quad bike double a arm front end with suspension also fitted to it. Also would like to remove the gov down the track and play with it a little. any help is appreciated and taken into account, cheers to all.

The 12 hp Briggs is plenty strong. Adding an alternator to the engine wouldn't be a big deal considering an alternator of any kind is self regulated and only puts a strain on the motor when you need power using something, other then that it is just idling any other time. But then again, no such thing as too much power with an oppey installed, LOL.

yeah I like the sound of the oppy's, the problem fitting an alternator to the oppy is it is physically too high to fit a pulley on top of to run an alternator, might need a bonnet scoop or similar to do so.

They made a fan that would work well with an alternator I think.  This came off an 18hp horizontal.  I am sorry I do not have any more info like model and type it came from.  I think it is called a flywheel fan.  I think it is similar to the older starter/generator style engines too.  I wonder how an alternator would work vertically? It would fit a 1/2 inche belt. I was going to try it as a pull start since it was notched for that, but it was just too big to get a good pull.

aha that would work I reckon, gearing might be an issue to overcome for correct charge rates but fitment would be cleaner, thanks for that idea mightyraze.