It is not imperative to convert to dual row chain if you've got a single row... it would be a good idea but is not absolutely necessary. The single row chain can be left in place as long as the oil is changed at regular intervals and the chain, chain guides and oilers are checked and changed if necessary but I wouldn't go more than 5 years without at least checking them. It is a lot easier to install a new single row chain than it is to replace the engine.Changing to a dual row change has become quite expensive. If you can find an engine, which is in decent shape, which already has it, it will save you a whole lot in both parts and labor.
Not necessarily toast. If you do a proper engine rebuild you will have a machine shop true up the cylinder walls replacing the pistons. You will also have the heads completely gone through. Now would be a good time to refresh the fuel system. And for heaven sake if you have a single row timing chain put a dual row in.Buy a used engine, your old one is toast now. $2500 is probably 1/2 of what it would cost to repair.
Yes what Walt said. I installed a great looking salvage yard engine in my Q45 years ago and it lasted just six months. Can't tell just by looking at it.You will get a lot miles out a proper rebuild than a salvage yard engine -- which you don't know how its been treated.
And therein lies the rub... I have every tool required to do the job and I love taking engines apart and finding what's wrong, sometimes I forget that the same doesn't apply to everyone.And therein lies the rub :
Not all here have the tools and knowledge / experience to tackle this job .
I'm a Journeyman Mechanic and I find bench work, like engine building, to be fun and rewarding .
Many don't and would not enjoy the basic in situ pull the heads and front of the engine to do the valve job and timing chains etc. .