I'd probably want a 15 or 16 ft head minimum but could run an 18 or 20. 13 might be ok for a year or so.

I currently run a 915 and am looking for a 920 on my 1988 Titan II 6620

1988 was last year made for the Titan II's so that is a plus on the one you are talking about.

Literature says 125 hp for the posi torq ground drive which would be the old belt drive variable speed. Hydro models are 145 hp

 Engine is the 466 in either case according to literature. 

Bu capacity of hopper is 166 but that is to the point of running over think you are pretty hard pressed to get that much into factory bin and extensions.

Good luck on the one you are looking at been a very good machine for me.

Here is some more info from the sales book.