Looking at your pictures again you might be able to get the existing assembly to work with a longer shock. Something a couple inches longer should get you to the proper bottom mount point without those stupid strap plates. A little research on shock lengths on different bikes should turn something up. The one issue with this is the link arm that's on there doesn't look like it is mounted right.

I think what you have is a PO who wanted a wider tire on the back. I would inspect the entire installation to insure it is safe given the botch job on the shock. How was the swing arm mount done? The F2 is wider at the pivot. I think the pivot bolt is larger on the F2 also. Did they just drill the holes out on the frame? How was the upper shock mount done?

 

this is the original OEM part
SHOCK ABSORBER ASSY., RR. (SHOWA)52400-MT6-671 is Not Available
Does anyone know by which I can replace it

 

Finding rear shocks is a PITA for these bikes. The only solution I have found was to buy an oldschool Fox TC and have it rebuilt. not sure of too many other legitimate options

 

There are options but they aren't cheap. Hagons, Penske and others. There are also rebuilders. Shocks from other bikes can be made to work also.

 

@dennisgb, the upper mount looks to be safely mounted as it was supposed to, not some mickey mouse job. I took the best pictures I could with the little daylight I had left after work. I live in an apartment and have no garage space so it makes things difficult. Im honestly thinking about buying the swing arm for sheer experience touching every part of the bike as possible. This obviously means sourcing some rims which kinda blows but whatever

@Zachy72, I already have the correct rear shock and link arm. I might as well go the distance.

Hey While we're at it, here's also a couple photos o fthe FRONT shocks. Since I have an F2 wheel, one can only assume....

 

Yes, I think your complete front end has been swapped too. Do you have pictures of top yoke and handlebars? That would confirm the matter.

Dennis has a good point about swing arm main pivot mounting. If the holes are drilled bigger, it won't be easy to swap back to F1 parts. And the pivot width too... I would like to see how it has been done. Might be another danger there.

I would try to fix things by replicating F2's rear suspension geometry as accurately as possible -> all pivot points should be exactly in the same positions.

 

Looks like an F2 front end. Looks like the PO wanted matching wheels at least.

I did a little research on F2 suspension on a F1. The geometry is completely different in terms of the pivot points. I'm not sure how this even works on your bike. Whomever did this was clueless on what they were doing which is obvious from looking at the side straps holding the shock. The back plate is there to hold the side plates from pivoting. This is a really bad way to do this considering the forces that the shock sees in use...it could be dangerous as well.

There is a thread on putting F3 suspension on an F1. The dog bone was cut and modified then re-welded to get the proper function and ride height.

I'm really not sure what you can do with this if the swing arm mounting on the frame has been modified. I'm pretty sure you will find that to be the case once you get into it.