Transition plate is the way to go.
I have welded to the aluminum cylinder before. AC capable TIG is a must and argon gas or argon with a little helium. I was showed a nice trick by a master at our garage. You should heat the aluminum object with a torch just before welding. Not real hot, not red, not even really too hot. Just good and hot. Then the aluminum takes the donor aluminum rod quickly and makes a perfect puddle. Worked like a charm. Before that, I had clumps of aluminum that were sitting there not knowing where to go. After using the torch to make it hot, the aluminum I added immediately went from spherical blob on the surface to being immediately absorbed into the surface with perfect unification. I recommend lot's and lots of practice welding aluminum before ever trying it on a useful part that you need to fix or modify. You will just ruin something. Even an experienced TIG welder that works with aluminum will have a better chance not to ruin it if they have a peice of the same metal to practice on...
I've made lots of adaption plates and avoid welding whenever possible. I cut wide strips of 0.8mm thick stainless steel and used them as clamps that the engine bolts to and connects to the frame. It doesn't require a very beefy connection at all. These are not dragster engines, lol)))