Had the similar problem a couple weeks ago, although I didn't have head replaced. Just a no start and A2 codes showed cam position sensor. You can check back through my posts or here is the short version of what worked for me -

Make sure plugs are not fouled and getting spark - distributor coil wire arc to ground shows spark. plugs you may want to remove and use propane torch to insure not fouled. I was getting close to due so I replaced.

Make sure battery fully charged.

Grind, without stopping until it starts. On mine took about a minute before I heard the first signs of life and the cranking started picking up speed and another 31 secs before it started. Total starter time 1 min 31 sec.

Got this from my indy shop who told me if you are getting spark cam sensor OK. He has had a couple 850s that were getting spark, fuel & compression and this is what he found.

Hope this work for you and not related to head replacement,

Good Luck

Ken