In short, yes it should be possible to lift the MD11C engine out using the boom, I did it very recently, on my 32ft long-keeled Cape Carib (a Ted Brewer design and the Hong Kong version of the Douglas 32) but using the main halyard to do the hauling isn't the right way to do it. The halyard should easily be able to support the load, provided it's greater than 8mm in diameter (the MD11C as a complete unit - ie including the MSB reverse gear - weighs in at 505 lbs, 230 Kg, or about two-and-a-half well-built adult males) but it pulls at the wrong angle to get the engine up the hatchway (if you have to extract it that way - for my installation, see the following URL: http://www.swainsons.com/gallery/dis...m&cat=&pos=-55
The way I did it was to use the topping lift to support the end of the boom, and the main halyard attached to a gybe-preventer sling to support the boom right above the hatchway. I then tied a piece of spare 10mm rope I had lying around in a loop of about 18 inches in diameter and laid that over the boom doubled. I used a 6mm diameter shackle to connect both ends of this loop to the cleating end of the mainsheet (previously removed from the traveller), and another larger shackle to connect what is normally the boom-end mainsheet block to a 10 mm dia rope passed figure-of-eight wise around the engine mounting feet (at the front end) and the mating end of the gearbox (at the back end).
My engine is installed below the bridgedeck and the galley work top. There is a "bridge" of bulkhead against which the companionway steps sit that the engine has to pass beneath, travelling in the forward direction, before it can be lifted out of the main hatch. This meant having to "walk" the engine forwards off its bearers, detaching and then re-attaching the hoist either side of the bulkhead "bridge".
Once the engine was free of the bearers and sitting on the (duly reinforced and protected) cabin sole, I used an overhead speedboat hoist to lift the engine out of the boat (this is because to work on the engine I had to get it up a 40 foot wall that abuts the pontoon, and didn't fancy trying to lug it up the steps. I *could* however have continued to use the boom to pull the engine right out of the boat and swung it outboard onto the pontoon (it would have heeled a fair bit of course - probably as much as 10 or 15 degrees, but this is down to individual boats' righting moment).
Some things to bear in mind, One: Please do remove all of the ancillaries before you remove the engine - by this I mean the alternator, starter motor, exhaust pipe swan neck etc - you'll make your life a thousand times easier. Two: please do remember to disconnect all electrical connections (including the earth) and the fuel lines from the engine before you try to move it - it's really annoying to start the heavy work, with your crew all ready to go, only to discover that you've still got something attached. Which brings me to Three: Please do make sure to have at least one strong buddy to hand to guide the engine while you haul on the piece of string, or vice-versa. and lastly: I have a 4:1 mainsheet tackle, and it's not enough to make this job easy. Either borrow one of those DIYer's car engine block and tackles, or take this opportunity to buy yourself three-gang top and bottom mainsheet blocks with a fiddle and a cam-cleat on the bottom block!
Also, if you plan to do the engine overhaul yourself, you'll find lots of pictures on the site above of my rebuild, and I picked up lots of experiences and knowledge that I'd be very happy to share. I can also help you figure the likely cost of replacement parts if you like - I had to spend HK$ 20,000 to restore mine, but it *had* been left for eight years with all of the cocks open, and at some point got sea water into it - eurgh!
Fair winds and big spanners