Unless you fabricate the 6 or so pieces of the 2 side sponsons, I'd say buying them would be a better option. The advertisements on this board usually have several sites which sell bits and you could try the 'swap' forum on these boards for the same bits. I'm guessing you need the upper and lower sections of the actual sponson and the center posts which holds the actual weapon.
As far as converting, once you get the necessary bits
ofc, I would not actually glue the parts of the sponsons into the hull of the
LR. After all the parts are painted, you should be able to just put the pieces into place w/o them falling off.
Another method would be to get some small 'rare earth' magnets and mount them into the hull and into the sponson pieces. 'Rare Earth' magnets are really small and powerful magents. I've only found them, of all places, in the cheap jewelry shops found in Malls. Usually they sell some earrings or crap like that for a dollar or so and you can just rip the magnets off the crappy jewelry. Maybe someone else will tell where a better place to get them at...I hate going into those stupid shops..