I did something similar to my old Load Rite which I call the "Franken-Trailer"
Way to short for my boat. New trailer the right size is about the same price as a nice long block. So what I did was go through the Load Rite factory catalog.
Installed a much longer tongue, and longer roller bars and added additional rollers on the rear bars, right under the engine. Did this over a couple seasons to spread the work out and cost.
The whole works is now about 25' long. Still a single axle but with a 6,000 lb axle, drop 3.5" and saddle style to get the boat lower. 12" Surge brakes, and 15" 225/75-15 tires. Added an additional brace where the tongue ties into the frame just like bigger heavier duty trailers. The frame was as thick galvanized stock as the heavier duty trailers so it all worked. I'm guessing the changes were about $2,000 total, which is less than half the price of a correct size trailer. I did it over several seasons. I got the boat as low as it can be, and the tongue weight is fine, even with the long tongue. The long tongue also helped with launching at our shallow ramp which thankfully is finally being replaced with a properly designed one! No one wants to put their rear tires in salt water!
BTW, if you have a single axle trailer but an HD axle (2"x3") adding 12" brakes gives you much better braking than a tandem with 2"x2" axles that have 10" brakes but only on one axle. The 12" brakes are rated for 6,000 lbs, the 10" only 3500. So I am a fan of a single axle but with a custom HD axle. That, 12" brakes and Load Range E tires 225/75-15.
Swing tongue can work as long as the material thickness that it uses for mounting is adequate. Some trailers have them from the OEM.
And despite the conventional wisdom the galvanized drum brakes work just fine for me. I set them with no drag, and honestly, I have had less corrosion issues than people with regular disc brakes on their trailers.
