89 Sunbird SKier, floor is rotten,Johnson 150 VRO engine

Hi,
I am pretty new to boating and I just purchased a 89 Sunbird Skier closed bow with a Johnson 150 V6
There are 2 issues that I discovered
1) Floor seems to be rotten is very soft and I am almost afraid to step to hard on it I would probably break it.
2) small crack about 2" on transom (left side on top) doesn;t look like this is something that happened lately.
I was thinking for this season to just screw a piece of treated plywood on the floor to cover the soft areas (almost whole back) and try to replace the floor next year.
There is a ski pole in the middle which is shaky and I don't think it would be smart to use it.
Now my question:
1)Is this a big deal to replace the floor? and is there a easy way to do it. I don't plan on having this boat forever but 3 years at least
2) should I try to to put a small vice on the transom just to keep it from further cracking?

The engine sounded great but I am not familiar with this engine either, any tips there?

Tanks for your help

 

Id put the plywood down for now. This fall cut it all out and rebuild the floor and glass it in new wood. Re work the ski pole in the under deck beams then . It all sounds like dry rot.
Might want to look into use some Boatlife GIT ROT

Id look into this a lot deeper See if wood in the transom is also bad like deck.

My .02 cents ED

 

thanks,will fo the plywood for now.
Would the treated one from Menards/Homedepot be ok?

 

Or any plywood that is for exterior use .

ED