Super Takumar score, and question

I've been downsizing a lot of stuff through the local freecycle list, and on occasion, finding something. Like this weekend, when someone posted a couple M42 Pentax lenses and stuff. I jumped on it, and now have a really nice Super Takumar 55mm 1.8, a 35mm 3.5, thread on hood for a 135mm, a Pentax polarizing filter, and an old Vivitar flash to play with.

All cleaned up very nice but for the 35mm, which had one very small spot of fungus on the outer surface of the back glass. I gently cleaned with hydrogen peroxide, but under the microscope, it's obvious it attacked the coating.

If I found one of these on ebay with another problem, say sticky blades or scratched front element, how fussy would it be to exchange the rear element? I can easily make a tool to remove the retaining ring. What I'm not sure about is if there's anything critical in aligning a lens element? Would it be a (gentle) drop in replacement, or is optical alignment something very critical? I've seen mention of marking glass elements so they can be put back in the same orientation.
But if it really is critical and the glass is coming from another lens, any such marks would be useless.

There's several examples on eBay in the $100 range, so thinking if I got a parts lens cheap, it would be worth my while to change out that rear element. Outside of that little spot, the lens looks almost brand new.

2nd time btw, I've seen camera stuff listed, last time I was a bit too slow and missed out. If you have a freecycle email list in your area, might be worth watching.
(now if I could just find someone giving away a Minolta 28-135.... )

Well, those were asking prices, and I was looking at really clean examples. Checking completed listings, $25-60 seems to be the range for a nice clean example without the leather case. And a couple went cheap enough to just buy and use outright. But if replacing that element isn't a big deal, I'll keep an eye out for a parts lens.

Still, the 55mm and polarizing filter were well worth the drive to pick them up.

Yeah, too often I make the mistake of looking first at asking prices, which often fail to sell. I will wait a while watching for a parts lens, as I just enjoy tinkering and fixing things up. I spent an hour machining a new plastic gear for a rechargeable flashlight I'd bought for $5. It worked great until a screw came loose inside and broke a tooth.
$45 shop labor to fix a $5 flashlight... but I had fun doing it!

Bob,

I'm really not sure what to think about the fungus issue. Most folks seem to walk, if not run away from a lens with fungus. Others disinfect and use. And found one person claiming the risk of cross-contamination virtually nil. I just don't know. So being cautious and heeding my brother's advice, I shy away from a lens with signs of fungus. If this lens didn't look so good, I would have just tossed it out. Willing to try replacing the rear element, as I believe it was moisture trapped by the thread on cap that got the ball rolling. There is no sign whatever of internal fungus, and hardly even any dust.

Nice to know about the 55, have to find some time to try it out!