Looks like you might be able to get to some of the clutch plates via the oil fill hole.
As I said in my previous post, just be careful and take your time if you try this method.
Thanks to Shyted for the photo.

All of you have given me options to consider.  If what MarkT says is true, and I'm not doubting it, I think I may go ahead and open the case and replace the plates entirely.  I figure if it isn't too difficult to do, instead of cleaning the existing plates I would be better off replacing them entirely while I'm in the motor.

I know saying something is always easier than doing something, but it seems once I get into the case, I'll remove the 5 screws and springs, then the pressure plate. At this point, all 13 plates can be pulled (I assume they slide out) and replaced.  The key is to make sure I replace them in the proper/correct order. Once the new plates are installed and in place, I would then return the pressure plate and secure it with the 5 springs and screws. Oh, and I will replace the case gasket since I will have it cracked open!

Is there any other advice I need to pay specific attention to?  Should I replace the springs too?  Is anything under pressure I need to be aware of?  Anything other thoughts?

I consider myself mechanically challenged, but I'm willing to give this a try.  Should be a fun project.  Now, I just have to order the parts!

And I've attached a picture of my HT1 today.  It has come a long way from 5 boxes of parts!!

That about covers it. There is a minimum length spec given in the manual for the 5 springs; if they are shorter than that spec, or very close to it, then it would be worth it to replace those as well.
If you are wanting to give it a try, you could open it up and try cleaning the fiber discs, and checking them for thickness (again, check the thickness spec in the manual.) while you are waiting for the new ones to come in. If they look good, and they clean up well, and you get it back together correctly, you have a start on your next bike when the new ones come in!
Remember, we can get you through this. "You can do it; we can help," as Home Despot's ads used to say.

Thanks Mothersbaugh and everyone! One question, when I begin to remove the 5 screws, should I strap my clutch lever to the handlebar to dis-engage it, or does it matter if it is engaged or disengaged?

I wouldn't, as it will make breaking loose those 5 bolts all that much harder, since tying the lever to the bars would be compressing those springs. You'll see when you get in there.
They (the plates and discs) should come out as a complete unit, and then you can tap lightly on them with a leather or plastic mallet, or you can try slipping a putty knife in between a couple of plates and tapping it lightly with said hammer.
When you get in there, it will all become clear.

Leave the handlebar lever released...  not tied to the handlebar.

Like I probably said before, the hardest part will likely be removing the cover and cleaning off the gasket.... speaking of that, one of the cover screws is almost hiding by the oil pump...  oh and you don't need to remove the pump from the cover.

Arrow shows the screw that is sometimes easy to miss with pump in place
 



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P.S.  I'd still try to break it free without pulling it apart.  I like gently prying with the screwdriver through the oil fill with the lever tied to the bar to try to help separate...  if it works, it will save you a ton of time.

Before and during taking things apart, always take photos as you go...........

Up to you of course & you're the boss. Thought i was the boss of 'me' but have decided my wife is lol.
Save yourself possible headaches & just ride it till it frees then give it some work. Cost nothing plus nothing to loose if it won't free but i'm sure it will. you've seen mine & bike looks terrible but did free. Even roadraced it lol & never had it apart, motor or clutch, ran great @ 55mph all day. Good luck with whatever you decide. Love the HT1's & remember the first one we assembled at work in "70, couldn't get over it--like a mini CT1 etc.

All are very, very good ideas and valued guidance. Thanks