Good question Ed...We're having the same difficulties! The intent was to make castings but after thinking about it, there just isn't time to make castings and build this engine at the same time. Besides, this milling machine was sitting there and it doesn't care if it is cutting air, wood or a piece of 7075.
Your engine sounds like a monster V twin - tell more about it....
Take a look at solenoid operated valves for an engine that size. The event timing can be tuned as well as the ignition advance curve. There are a couple of sites on the net by experimenters in this stuff. The heads would be simple and there is no cam - what a savings in power. There is enough sensor 'garbage' in junkyards to do most any crankshaft position sensing you would need. I'm currently dabbling in a programmable ignition system using a PIC chip (which have become fast enough for just over 10,000 RPMs) that changes the advance as the speed increases in various curves and operating the valves would fall right in there as well.
Time; I operate an electron beam welder at work that is enclosed in a six X six X six foot cube cryogenic chamber and there is vacuum pump down time (down to 7E-5 Torr) and then part cool down time between welds = time. I usually spend much of that time doing CAD work on various projects for the lab (and myself) so these CADs of the HD engine and NC programming that take lots of time are conveniently done at work. That leaves home shop time in the shop! But today (Saturday), I'm at work. We have had problems with the cryo system so I've lost welding time and many parts have piled up to do.... Need any CAD or programming done?
Bill C.