Maybe switching to an electric fan would free up a few HP. Not sure how much that would net you. That is if these don't have an electric fan already, my 2014 doesn't and I wouldn't think they'd take a step back in technology from one engine to the next. I've been surprised before though.
Then there is the possibility of doing some under drive pulleys if any are available. I'm sure some were made for the car applications the engine was used in, they might fit.
It's a little "Fast and the Furious" sounding, but you could port and polish the intake. Make sure the intake to head gaskets are open enough so they don't impede air flow, same on the exhaust side. Port the heads. Get a valve job done. If you go that far, while the cylinder head is off you would want to have it machined anyway, it would bump up the compression just a tad, but nothing really noticeable.
I don't know what kind of extra HP that would net, it's just some of the basic things I could think of that a lot of people used to do back in the time when I was interested in these things.
My concern with adding a turbo would be the bottom end longevity. Would the crank bearings be able to handle it?
You may be able to accomplish more with a simple gearing change to the rear end. A lower gear ratio would give it the feel of more power, have a better take off, but definitely cut the top speed.