Wow, she looks like a tidy ole girl… I’ve got to give ya a thumbs up... your first bike and you have caught the bug and are going about ripping it apart and making it something to your liking.
Advice on do’s from me is make it look like whatever you want, it’s a personal choice and those that may not like it... don’t matter.
I’ve used SuperTrim for seat upholstering before, the job was ok... not perfect, and semi reasonable in price, Mike Dodd (a member on here) has the contact details for a guy down South which seemed pretty good… others will hopefully come forward with more contacts.
As for what you can get away with before having WOF issues… In my experience, you can actually do quite a lot… I say in my experience as the WOF place I go to are really good and they ride bikes themselves… I did a bobber and cut the rear frame section shorter by about 100mm, welded on brackets for indicators and mounted a completely different rear guard and single seat with no issues at all.
If you’re thinking about changing the mounting points of the rear shocks, or cutting off the rear frame section completely or the like, then you will have WOF issues.
If in doubt, ask before getting stuck in with an angle grinder.
If you have any more questions, fire away… we like to think we’re a helpful bunch.
I’ll be keeping an eye on your thread, I might be looking at powder coating a frame too… for me it all comes down to the cost as I’m doing a bike up with the intentions of selling it…. Though owning 1 more bike couldn’t hurt?