mate your frame is bent... new frame, different swingarm setup, then you may run the twin pipe setup, but ONLY if you run a non-external reservoir shock, or a remote reservoir rear shock... eg DNM MT or MST-BAG....
seriously, to convert the a-frame swinger frame to a monoshock swinger type frame, you are looking at a complete cut and shut job, and frame rejigging.... by the time you're done with all the fu**ing about, you could have bought a better frame....
and no, you cant do the mods to the swinger that you've mentioned. will completely change your leverage ratios, and you wont find a shock that will have spring or valving rates that will work with it..
Hate to say it mate, but i'd turf the bike you have, with the exception of the motor, and start from scratch...
good mods? first thing to try on ANY motor, is carby and exhaust.... Try an OKO26mm Flatslide...
exhaust, well, it looks like you'll need to get a new frame to suit the twinpipe setup now dont it...