Rectifier is just a couple diodes that divides the AC wave into DC+ and DC-  I recently had to replace one on an old Honda CT90. The original one was this bizarre looking stack of plates. I just went to the local electronics shop and bought a 5-10 amp silicon rectifier. I think it costed $1-$2. 
It worked great. 

PMB that reminds me of 50 years ago at work. The CT90 had the light switch bringing in extra coils through the multi plate rectifier. We had a faulty alternator on one shorted to earth & customer had no money for repairs. So i un-earthed the multi plate rectifier then hung it in a rubber casing & it worked because the stator 'was' earthed through a winding fault short in one of the coils--but shouldn't be. Was just my desperate attempt to keep some charge in the battery for him but charge plenty. Those rectifiers were earthed via the mounting center bolt. Mind you, that was 50 years ago & doubt its still alive. System was originally opposite to Yamaha system with rectifier not earthed & lighting coils earthed. So thought try it the Yamaha way haha. No regulator involved with those hence the switch bringing in extra coils to counteract the lights when on to keep the charge going.

I see thats good to know because i was gonna throw the regulator on and forget about the regulator. The 75 DT250 that im doing has the original on in but im not sure if its any good or not. Is there anyway to test those? also do you have any recommendations on which one to buy?

Is it possible that the rectifier mounts on the stud on the bottom of the battery box? The appears to be the area where the connections are.

Unlikely - the flasher relay mounts there (should be a brown and a brown/white wire exiting the main loom in that vicinity).

 Here is some pictures of one of the RT3's i restored. It mounts under the tank on the welded block right in front of the coil. see pics. I have 100's of pictures of the RT3 when i restore this one. the 2nd picture show the block that it mounts to on the frame.

It was a special restoration to say the least!  

  

I agree with SKYDANCER46 that it was a special restoration. I ended up with it. Here it is in front of 2 of my others.

 
 

Isn't that called "hoarding"?  
  Those are some very sexy bikes! I'm drooling now... If I can git my RT2 close to that.. I would be very happy!
Sneezles61

I have a '68 CT90 which I restored and really love, despite the motor being 200 cc too small for me!  I also replaced that selenium stack with a full bridge rectifier, to your point is was like $2 and worked like a charm!

Sneezles61 said: Isn't that called "hoarding"?  

No......it's not hoarding,.... just stocking up in case of a shortage.