Putting the bike on the centre stand and using
one of these (with the cups placed on the bottom run of black frame, assisted by perhaps some thin strips of square wood if needed) will help you lift the front of the bike, if needed, to complete the work. I've got both a lift like that and the bike but haven't checked if the thread on the cups are long enough to ensure the ramp touches the frame first, and not the exhaust pipes, when raised.
Or a headstand.
Or if you've got some ratchet straps you can tie them to a strong beam in a garage or a tree and keep the bike raised up that way (putting the strap around the handlebars).
~
As an aside, and encase you don't know (forgive me if you do):
> the rear caliper is from a FZS 1000 (the brake caliper bracket will be slightly modified to make it fit)
> You can get a Torsion bar (the black bar attached to brake caliper bracket) from a Yamaha Thunderace (of better quality and wont rust as much), if ever needed
> custom seat
> aftermarket handlebars
> you're missing the R/H cover that goes over the airbox
> Rear disc looks questionable, but front looks to be EBC?
> Might want to look at that ding in the R/H header pipe
Bikes either had an owner that pumped loads of money into it (rear caliper upgrade, ebc discs, mudguard etc) and then one that ruined it, or one that suffered conflicting upgrade choices and has left you with half decent half project.
I only post this bit as it seems like you've just purchased it and are attempting to figure it out. Parts of it are definitely not standard but fairly easy to work on.