Chris, there's nothing to stop you welding or bolting a rectangular base on to the existing trailer which would be far better for strength and stability, wont affect the boat if you place it right. The trailer in it's current state isn't suitable imo remember a boat only rests along the centre bar so as Dabop says you need to look at weight distribution.
However there are several things to bear in mind. You need to know the weight of the trailer as it stands now, it looks quite beefy, you also need the weight of any extra metal, timber etc you add whether fixed or temporary and need the combined weights of the bikes, you mention two or three. All these have to be added together and remain below the 750kg weight limit for an unbraked trailer or you risk a fine. 750kg sounds a lot, it isn't!
I gave the weight of a fairly common 125cc bike up above (a quick check shows most are actually a few kg lighter) so two bikes would be about 280kg, leaving 470kg for the trailer- I know my own boat trailer (which is a center beam plus 2 shorter side beams, triangulated at the front- with the suspension mounted on a seperate rectangular section that can be loosened at its ubolts and slid back and forth to achieve balance) weighs in at only 170kg!!!
Not the best shot (I happen to have its compliance plate stored on the computer for the insurance, along with all the other plates off the other trailers) but you can see in the shadows the center beam and two triangle beams in this shot
His existing beam and axle would have to be solid bar stock to make up THAT much of a difference!!!
Actually I wish mine had a higher ATM, as it is currently only 450kg- which means when I have the 40hp on, and a full tank of fuel for it, it is VERY close to its max legal!!!
(back when this trailer was made, the rego cost more as the ATM rose, so people would make it as light as possible to save money on rego here)- now its the same price for anything under 750kg here, so a full 750kg on it would cost exactly the same as my current 450kg... (that 170kg trailer is long enough to fit my 4.2 boat on btw, so its not a small boat or trailer)
On thinking about it, I might get it reclassified to 750kg, it would mean a new rego inspection and a new weighbridge ticket for the DMT, but would add peace of mine when the boats got the fridge and other junk in it...
When I bought the boat, it still only had a 9.8hp on it, and my 40hp is considerably heavier (I can carry the old 9.8hp by myself, the 40hp took two of us to lift it on, and we were struggling with it!!!), which is why its so close to the limit- as it was I had to slide the axle back further to actually get downforce on the ball!!!- When I first put the 40 on, it was sitting on its jockey wheel, and it lifted the wheel off the ground until the motor hit the ground!!!
Not good!!!
I also swapped out the little wheels for a set of 14" car rims and tyres, as I towed it long distances (sometimes thousands of km on holidays) and I have seen WAAAY too many of those small wheels spin off over the years (remember they turn a LOT faster than your car tyres do to cover the same distance as their diameter is smaller)