My bellypan has been off my bike for quite some time because the two rubber/steel vibration-damping bolts have fallen apart. I just noticed that I have a couple of stainless steel bolts that match the size/thread of those bolt-holes on the engine, and I'm fairly certain I can use them to reattach the bellypan. 
Is there any reason why I shouldn't just go ahead and reattach the bellypan with the tools I have on hand, or is the original rubber/steel bolt necessary for proper fitment/function?

M8x1.25 as I recall.

I wouldn't simply bolt the belly pan up tight to the sub-frame. Some sort of spacer so the bolts can be tight without crushing the plastic. Then a grommet or bit of rubber tube to stop it rattling. As far as bolting up the sub-frame to the engine, spacers are required to bring it out to the right distance. On my 16V it's hard on the sump on the left and spaced out on the right. But correctly spaced, bolt it up tight, rubber grommets not needed.

The correct bit are available and space the plastic away from some of the heat, keep the stress or the bolt holes down and make a nice job of attaching the belly pan. For me at least I use what was intended.

Also a little movement in the belly pan to prevent stress cracks over rough surfaces or contact with rocks.  

Here in Oz we have a few goat tracks that pass for main roads and my K sees a bit of dirt road as well.  On our recent RossKKo Memorial trip I broke the main fairing bracket due to vibration as the bitumen road was that bad.

Thanks for your advice, folks.
While I'm moving forward with the bolts I have on hand, I did go ahead and pick up a couple of large marine-bolt rubber grommets, which I then belt-sanded down to a usable thickness. These are set in place exactly where the original rubber would have gone. Likewise, I used a series of rubber seals I had laying around to serve as a secondary line of vibration damping on each side of where the bolts fasten the armature to the engine, as well as to where the bellypan faring attaches to the armature. ...and now that I've got all that worked out, I have to take it off again before doing a test-ride, because I need to do a transmission transplant first.

KentM wrote:Thanks for your advice, folks.
While I'm moving forward with the bolts I have on hand, I did go ahead and pick up a couple of large marine-bolt rubber grommets, which I then belt-sanded down to a usable thickness. These are set in place exactly where the original rubber would have gone. Likewise, I used a series of rubber seals I had laying around to serve as a secondary line of vibration damping on each side of where the bolts fasten the armature to the engine, as well as to where the bellypan faring attaches to the armature. ...and now that I've got all that worked out, I have to take it off again before doing a test-ride, because I need to do a transmission transplant first.

  

I have a FlatRacer belly pan complete with their stainless steel mounting kit. They do sell a stainless steel hardware-only set for mounting YOUR engine bracket and YOUR belly pan to that bracket. Just thought I'd mention this even though it sounds like you've got yourself all sorted out.