Please do not just bung in a battery, tip in fresh fuel and try to start a machine that has sat for 20 years. In fact even "dry" cranking it is not recommended, the reason is that every bit of particulate matter that was suspended in the oil when it last ran is now sitting in the bottom of the sump just waiting for the opportunity to be picked up by the pump and be sent on its way around the engine.
Do the following.
Drain as much oil as you can from the sump and then remove sump - I suspect one look at what you will find will convince you of what a good idea it was to remove the sump.
After cleaning the sump - refit the sump with a new gasket, noting that these gaskets are designed to be used with no magic potions or lotions on the mating surfaces, the next person to remove that sump will thank you.
Change the filter, and along the way read up on the $2,000 O-ring and make sure you don't become a contributor by measuring your filter canister depth and using the correct number of shims and the gasket if needed.
While you are in an oil changing mood, gearbox and final drive as well, with the correct grade and type of oil.
The brakes have probably gone South as well in 20 years, before offering advice I enquire if these are something you will fix yourself or will you send them out?
Back to the engine. After cleaning the tank out and filling with fresh fuel I'd turn the fuel on for a brief period just to see if the carbs are going to instantly piddle all over the floor. if they don't I'd leave them be for the present, but I;d order a rebuild kit and pencil in a "carb day" in the near future as whilst they will probably get you a trial start and run, they will most likely not be anywhere near rideable condition.
To return to the engine, With nice brand new oil in it now, but the bike on the centrestand and remove the rocker covers (and I'd have new gaskets handy just in case - new rocker cover gaskets are just something you should keep anyway.
Now remove the sparkplugs and reconnect them to their leads and lay them on the cylinders in a way that you can see the hot ends of them.
Crank the engine over until you see oil flooding onto the floor from all 4 rockers - no oil, stop what you are doing and investigate why. In 20 secs or so cranking you should get around 150~200 mls of oil if the oil system is in working order.
Whilst cranking, do you see sparks?
If all that looks good, refit plugs and rocker covers and "go for gold". If it starts, keep the revs fairly low and listen for any expensive sounding noises.
All good? After the brakes are done and the fluid and seals change dint he front forks you are ready for a gingerly test ride on your 20 year old tyres that must be throw away pretty soon, but will do just fine for a low speed ride around your suburb (or two).
More than likely the clutch will have frozen, don't despair, they are usually fairly easily freed up, but pencil in a day for spline lube, rear main seal and gearbox input seal rehab. And yes there is a rear seal on the gearbox you may have to change as well, but it is a bear, requires special tools, so if it isn't leaking now you can ignore it for a while.
Lastly, photos please!
