Here are some background notes about the Viebach ST05G Stirling Engine, the original prototype and the construction. Basically a DIY Stiling of about 500W shaft output based on aluminium casting set.  Power output improvements to 1kW should be possible. This engine has been built by amateurs – mostly in Germany, and I have seen it run on wood, charcoal, wood pellets, natural gas, propane. A ready built engine can be bought for $4000, or home built much cheaper.
I first came across this in 1994/1995 and travelled to Munich in October 95 to meet Dieter Viebach. We shipped over a complete micro-cogen unit built by Barney Scharl and his brother, and Helmut Scheffers – key members of the “Study Group Stirling”  This unit was demonstrated at the Model Engineering Exhibition, Olympia London, over Christmas 95.
The original Viebach prototype was fabricated from welded 6” bore steel water pipes.  The crankcase was two pipes set at right angles to make a classic T-shape crankcase – just like a model aeroplane glow-motor – only much bigger.  This arrangement  was cheap strong and plenty of space inside for crankshaft and linkages.  Someone in the water industry may know of a large (6” bore) T piece or similar shaped valve unit, which could be used directly.
The first prototype was shown at the 5th International Stirling Engine Conference (ISEC) in Eindhoven in 1993.  Following this showing, Viebach developed the aluminium sand casting set to simplify construction. The castings are good quality but you will need a big lathe ( 8” over bed)  to handle them.  Milling of the crankcase may be a preferred route.
The cooler is a double walled assembly about 7” diameter and 4.5” high which flange bolts directly on top of the crankcase.  Above this is the hot-end assembly, again flanged on.  Viebach uses a standard flange pattern for all his parts – good idea for different engine configurations.  The hot end is fabricated from stainless steel (316L grade  – or 18/8)  It houses the regenerator and has the heater head attached.
Barney Scharl adapted the domed Viebach heater to make a simple bolt together circular flat plate in 3/8” gauge with tubes welded in.  This approach really simplifies the top end construction.  The regenerator was made from stainless steel swarf –  kitchen scourer pads, pressed in with a bench press. The regenerator housing could be made from stainless steel flue-pipe – available in standard sizes, just weld on standard (Viebach) flanges at each end.
The displacer is fabricated, but you may find a bit of 4” diameter stainless flue pipe or even a stainless cooking contained (Asparagus steamer) which may do the job.
The power piston and cylinder were originally fabricated by it would be cheaper to source these from an oil free compressor or small IC engine. You may also get the conrod as well.  It should be possible to increase the power cylinder bore from 3.125” to 4”  and the displacer from  under 4” (96 mm) to about 5”.
Viebach gets 1.3W per cc of power piston swept volume when operating on air at 150 psi  (10 bar). the basic engine is about 377 cc  – 80mm bore x 75 mm stroke.
These castings can be used with a high degree of flexibility allowing freelance designs.  Increasing bore and stroke, and changing from gamma type engine to beta or alpha type has been accomplished by Dieter Viebach (& sons) and Bernd Kammerich. 
The maximum that you could hope for from an engine based on these castings is 1kW of shaft power.  The Viebach design, has been proved to be safe to 150 psi. I would seriously caution the use of air pressures  higher than this – in any amateur built Stirling, because of the risk of lubricating oil exploding in the hot end – Goodbye Stirling, Hello Diesel!