In lines have been powering models for quite some time. 3w, Taurus, Sachs, and a company in Canada have made them for a very long time. I haven't seen any Vee types, though I believe them to be out there.

The problem with "non standard" cylinder arrangements is cost. Relatively easy to find cranks and make cases for opposed, but nearly impossible to find cranks for other types. Modeling in actually only a very small percentage of the gas engines sold for use so if another inductrial application is not readily availble nobody would bother with products considerably more expensive to make. Modelers would not buy enough of them to justify the expense of manufacturing. 10 or 30 engines a year would not fund a business or maintain a production run. Radials are another that sell very few for the effort and expense involved with their manufacture, which explains the higher selling prices.