One cautious plan of attack would be to try starting it 'as is' but not without some preparation. Full oil flush and confirm circulation; Fresh fuel, clean filters; confirmed ignition. Here is my method.
Remove the seat then the fuel tank. Once off, drain the fuel tank, then bit of fresh fuel and a good slosh about to remove any crap from the inside. You may consider removing the fuel taps as they most likely will have a gauze filter on the bit that is inside the tank - and it will need to be cleaned out. Clean or replace any inline fuel filters.
I would also remove the fuel lines from the bottom of the carbs so draining the float bowls - it would be good precaution to also remove and flush out the main jets. Flush some fresh fuel thru the fuel lines. Do not refit the fuel tank just yet.
Next drain the oil from the sump and also from the oil tank. Also drain the oil filter chamber .
At that point, with the line that connects to the inlet of the oil pump disconnected I, would put fresh clean oil into the oil tank till such time a clean oils is flowing out of the pipe that's at the oil pump inlet. Now refit the connection to the base of the oil pump and top up the oil tank so that the oil just reached the cross bar you can see when you look down into the filler neck. Now with a fresh oil filter element in place and all oil lines buttoned but the sump plug out, up squirt oil into the quill till it starts to appear at the sump drain hole. Put the sump drain plug back into place.
Now squirt a good half cup full of fresh oil down each of the 4 pushrod tubes. Also remove the lock dome nut on the top of the timing cover and the jet below it and put a few good squirts of oil into there - take it slow here else you may get oil everywhere. Now put those bits back together.
Remove the spark plugs and with the bike on your rollers - I suggest in 4 gear (which is direct drive 1:1) get the motor turning over for a couple of minutes. While doing this have the cap OFF the oil tank so you can see the oil being returned. It may take a minute or 2 to start being returned.
If there is no oil being returned then there is an issue with the oil system that MUST be fixed before you attempt to start the motor.
I expect the returning oil will be very dirty. You can keep this up till the oil appears to be coming cleaner when it will be time for another full drain of the oil tank and a new lot of fresh oil into it.
Now you know oil is fresh and circulates. (BTW after your first 100 miles of running do a complete oil change and fit a fresh oil filter)
At some point here you will need to remove and clean the magneto points, setting the points gap to 0.012". Set the spark plug gaps to 0.018". I suggest new spark plugs .
Now you need to check/set the ignition timing - What's needed for easy starting is 4 BTDC fully retarded and for running 32 to 34 BTDC fully advanced.
At any point around this time check the 'tappets' - actually they need to be snug with zero clearance with the motor cold but you should be able to rotate the push rods by hand.
Stick a spark plug into each of the leads (not in the motor) and again on the rollers turn the motor over looking for a spark in each plug.
If all is well fit the spark plugs into the heads and get some fresh fuel into the carbs - use a shop fuel supply or refit the fuel tank.
And now the moment of truth - will it start!
Next steps involve getting it road ready - brakes, tyres, electrics, suspension and so it goes.