I'm pretty sure it's a do it your self'er.  I'm intersted to hear others opinions as well as its a project I'd like to do as well.  Are you planning on going with a composting head?.....Oops, just saw that you already have the composting head, which type is it?

Dave

I replaced the entire toilet system in our boat some years ago.  some thoughts and experinces:

• Replace all the hoses,  Beware the pump out hose may still have stuff in it,  There WILL be a mess.
• You'll ned to cut open the framing around the tank to remove it.  Putting the framing together will be the very last thing you do.  We bought a Ronco b167 tank.
• Deciide if you want to replace the toilet, I needed to so I bought the "identical" Groco HF manual toilet.  The mounting spacing changed only 2 or 4 lag bolts were in the same place.
• Decide on the guaging you want.  I went with the FireBoy-Xintex LLM-1 w/ FS-12 Flange Sending unit.  I think I should have gone to an  external sonic style sensor.  The crud causes the sensor pick to be unreliable.
• This is IMPORTANT after all the hardware, tank and plumbing is done and before you glass in the framing.  Fill you tank above the drain hose fitting with colored water.  Use red food coloring dye and place clean dry paper towels under the drain fitting in the bildge.  Come back a day or two later.  If the paper is wet and red, the tank and fitting must be redone.  I happen to me.  You don't want a black water leak.  After you satisfied the tank is not leaking, glass in the framing.

It is a two person job, some swearing is to be expected and hold off on the beers until the job is done.  A nice to have is device called   Holding Tank Eye.  It is a visual inspection device, no moving parts and I tells you how much is left in the tank.  It gauges the last 3 inches in the tank.  The surface area of the tank turns out to about a gallon per inch.  My electronic guage shows 3/4 or full.  The sight guage is very useful.

Lou Bruska
Sojourn
1985 C-36 Hull #495