What would the implications be for doubling the size of MB22. I would like to build one but the small tooling and bolts required are expensive. If standard off the shelf bolts, taps & dies could be used, it would be possibly easier to build and cheaper.

Hi.  Without knowing what you have in the way of experience, equipment and tuition its difficult to advise on your question of scaling up.  Its certainly possible, but choosing another design may be a more sensible solution, as port sizes etc will have been optimized by MB for an engine of this size.  They may not be the best size if doubled up and you will still have to decide on what size fixings to use.  The latter would probably not need to be doubled up to be effective.
Have a look, if you can, at John Bertinat's "Marcher" & "Border" designs.  Though intended to be built from castings, most parts can be fabricated or made from the solid.
If this is your first engine a larger " V" Oscillator, like that supplied by Blackgates may be a better bet.
As always, you will find joining a local Model Engineering Society more than worthwhile.  They usually have a wealth of experience to share, a library of Model Engineer etc and collections of drawings.
Good luck with you project.  Mike