i would like to find out if u can fit an 80hp outboard motor to a 4.2 metre stacer runabout

 

That sounds like it might be legally overpowered. Does it have a horsepower tag? Another thing to consider is when you slow down with a big motor on the back of a little boat the bow goes up and a big wave comes over the back and can swamp the boat.

One of my boats is overpowered and I have to be carefull as to when I take it out. It can't be windy and I can't take it to lakes with a lot of traffic as the back of the boat is 6" above the water line. I also can't slow down verry fast or I get wet. I do have 2 1500 gallon bilge pumps in it to get the watter out fast and I added a lot of flotation just in case.

 

According to Stacer's website, the max horsepower of the 4.2 is 40. The max recommended weight of the outboard is 110kg, which is well under the weight of a Mariner 80.

In addition to samuari's caveats, overpowering a boat (especially an aluminum boat) by 2x can literally rip it apart from the stresses to the transom & hull.

 

I should have looked at what type of boat that is. I would NOT do it. 2x over power on any boat let alone a aluminum boat is a bad Idea. This is in no way a performance boat.