Is my iron useable?
 

no

Wow that rotor and rotor housing are unhappy!

On the rear iron, my rule is if your fingernail catches on it it's too big. Then it goes to budget, if it makes sense to buy a good used or new iron.

But, that iron looks like it has sanding marks on it like someone sanded it down at some point. That worries me more than the groove.

That iron may result in a little less engine life and/or a little less compression, you have to see if it's worth it cost-wise to replace to get that bump. In general I don't think that iron will cause major problems. If before the motor blew you had decent compression, no smoking, good idle, etc. you should expect that and more after the motor is rebuilt.

Dale

Lots of scratches on that iron. It could be lapped and renitrided but it becomes a question if its worth putting the money into it over a good used or new iron.The exhaust port on the rotor housing also looks like it lacks a bevel on the closing. That will eat up your seals and cause the smaller scratches I see.

Try to get decent used irons. Don't use them.

No, just buy a new iron. 450 is the least of your worries in a full engine build.