fuji
I,v seen all these claims for guarantees before and they are usualy only honored when the engine is retuned to the overhaul shop.
If the shop is in the USA then the cost of removing the engine and refitting ,as well as the shipping costs make the guarentee worthless to you on this side of the atlantic.
The Lycoming factory guarentee may not last as long as the other shops but it covers ALL the work needed to restor your engine to working order including the removal and refitting and shipping if required.
As most problems will show them selfs with in the first 200 hours or so the chances are that the lycoming guarentee will cover you for this.
Having had to make guarentee claims for owners in the past I have found lycoming to offer very good costomer support unlike some that I wont mention here.
I have had no dealings with the millenium engine company and could not comment one way or the other on the quality of the work or guarentee they offer but I would advise you to READ THE SMALL PRINT in the contract before you commit your self.