The rifle has been acting funky lately, seemed down on power. So built a compressor and removed the mainspring, and found it was broken about 3" from the end (barrel end). I've never messed around with airguns, so at this point, I have a few questions:

1. Replace the spring with an OEM, or buy a tune kit?
2. What is involved in a tune? Kind of hard to find exactly what to do, where to apply grease, etc.

I'm leaning toward just replacing the spring for $20, but if tunes are easy, and worth it, I'll upgrade to an $80 kit.

Thanks!

Jim (the Springman) Maccari will sell you a higher quality spring for $20.
Or a fitted spring/guide kit for $80. No more vibration.

It's been a while since I had an R9/R10 apart, can't remember for sure if the breech is threaded or pinned in place.
IIRC it's just pinned (threaded breechplugs can be difficult to start without crossthreading).
Your broken spring can probably be removed without a vise, and some aftermarket springs allow hand assembly.
You probably aren't putting it back together with a new factory spring without a bigass vise.

The R9 breech is just pinned in place.  A simple rotation and it slides right out  And yeah, I found that with the broken spring, I didn't really need the compressor.  But that's OK, I've got it for when I put in a new one.

I'll probably go with a kit, and get new seals in it also. It's probably about 15 years old, and not been fired for a couple years.

A vortek kit is another alternative.  You can call Tom, the owner of Vortek to get straight and solid answers.  Maccari makes it clear on his web site he doesn't exactly want to deal with amateurs.

Please Note;

Our target audience is skilled gunsmiths and airgunsmiths. So we supply no instructions or advice on assembly, dis-assembly or exploded diagrams or "how to" books or files. Thank You!

Jim does expect one to know what they are doing with his stuff.  But, there are a lot of options with seals and spring constants and preloads that he can supply.  There are other sources for springs as well, but you will need to cut to length and close the ends.  Then there are different kinds of spring guides available as well.  With all these options, one can tune a spring gun to be a much better performer.  However, much of it is trial and error, and keeping records of what changes do.

If you don't want things to change much (and the R9) is pretty damn good as it is, just replace the spring with the exact same one along with the piston seal.  Things like the Vortek kit will change the power and behavior, which can be an improvement or not.  

Some instructions say to grease everything with thick coats, but from what I've found only a light coat is needed wherever metal contacts metal.  More grease just attracts more dirt.  A light coat of moly is recommended for the seal bearing surfaces.  Avoid oil containing grease on the seal and especially in front of the seal.

I've used both, I like both.   I am running a Vortek tune kit in an old Diana 36 and one in my TX200, both easy to install but you really need to put some pellets downrange before they settle in.  I have a Maccari kit in a old BSA, brought that sucker back to life.  BTW, I retired my home made compressor for the Vortek rail lock, that thing is great.