You can remove the spring, in situ. Something similar has been done on the side of the road. It is not easy and you might miss something doing it this way. Additionally, it took two other riders to help get it done.
That being said, you will likely need to replace the inlet spring. The springs do not last very long, but they are run very long...almost forever. I bought a bike once that had some power issues and I could move the springs by hand. The head came off and I replaced the springs.
It sounds like the same thing. If you did not break a spring, or it broke as it shut down, I suspect all is well. I also had a Vincent drop the valve guide and the ex stayed open just enough to do roughly the same thing. This tends to happen right as you are shutting down when the engine is hot.
The first time is always the worst, but racers tend to take the UFM and forks off in one piece, quite often and can do it quite fast. It is more difficult with a wiring harness. It is nice if you can do an overhead lift of some type just to steady the UFM and forks.
So, by the side of the road you put the piston at bottom dead center, or close. Cut one end off a bungee cord and slip the cord itself through the spark plug hole. The other end with the clip on it will stay outside the engine. turn the engine to get the piston toward TDC and the cord will close the valve. At this point you can do what you wish with the spring. In this case, the circlip on the valve flew off and it was reinstalled while others pried the spring and retainer down.
Best of luck,
David