Need nore info. What year is the boat?... bracket, notched transom, etc?
Are you sure it's an "Offshore" model? Never heard of a "24.5" Offshore.
The last year for the Offshore model was 1991, then it became the "Explorer", which ran up until 95. the Offshore and Explorer models had the same basic specs. Length was 24' 1" and a max hp rating of 300, (actually, pre 86 Offshore models had a max hp rating of 280).
In 96 the Explorer was replaced by the Voyager. It is a completely different boat with a length of 24' 9" and a max hp rating of 350.
The reason I mention all this is beacause of the "twin 175's" you mention. Again... doesn't make sense for an Offshore model, so maybe you have a Voyager?
If it is an Offshore or an Explorer, yes... you can get away with a single 225. It is enough power for that hull. No speed demon, (a 250 would be better) but ok. And again, without knowing the specifics, (bracket, notched, etc) it's tough to say how difficult the replacement/installation would be, (going from twins to a single).
If it's a Voyager... no, I don't think a 225 enough power. The hull is longer, wider and around 500 pounds heavier.
If it were me, since the boat's already set up for it, I would stick with twins... much better capabilities. Imo, ideally, you want the hp to be as close as possible to the max rating, but I wouldn't exceed it.