Wanna know how you can tell you are dealing with a schmuck who has no idea what he is doing? This. Read the initial post.
Ok Benny, I'll oblige.
First off the sending unit for an airspeed indicator on an airplane is called a pitot tube. remember that phrase: pitot tube. in order to use this you will need one. One from a spit usually sells for between $160 and $230 just because it is from a spitfire. I have never seen one from a lancaster for sale.
First disappointing part - you can't use your gauge to convert to this type of reading. You are going to need the whole assembly - the pitot tube and the actual airspeed indicator (they are a matched set). If I had to guess what this would cost to use one from an actual spitfire I would put it at $1000 all said and done not including installation.
Second disappointing point - it needs to be in clean air. This means you can't have it hidden someplace on the bike. If it were me and I was concerned with how it functions I would have it on a 2 foot stalk coming out the top of the headlight housing, i.e. it would be the most forward thing on the bike. I would sugest the front fender but you would have to run it with the lines exposed.
Thrid disappointing point - it isn't road legal. Ever been in a taxi-ing aircraft? the gauge doesn't read below 10 kts. Oh by the way, airspeed indicators are in KTS, not mph (although some do have ground speed markings on the gauge as well).
Fourth disappointing point - airplanes are 24 volts. bikes are 12 volts (sometimes 6 volts). getting one to work correctly would require some creative electrical work (of which I have no idea).
Here is some saving grace - the way they measure speed in light aircraft hasn't changed since the airplane was invented. That means you can probably salvage all you need off a Cessna 150 or a piper cherokee and it will cost you a fraction of the cost, work better than 40's junk, and nobody can tell the difference except really good airplane buffs.
Why is it a stupid idea though? because it is adding 15 lbs of crap to a bike just to measure speed in a way you can't actually use. I mean if you were racing at bonneville and were expecting to be in the 200mph club, that would be one thing, but it sounds like you just want to impress everyone in your local starbucks' parking lot.
want something better? how about you call up VDO and ask them about their automotive gauges that look like airplane gauges and have them make you a custom one that works with an electric speedo unit. Then find an electric speedo sender off a modern sport bike that fits your bike.