In which case, Chapman's Architectura Navalis Mercatoria of the late 1760s might be of some help. Other than the positioning of the masts, the deck layout of such a vessel would be more or less identical to a contemporary brig or snow. Plate 61 (the one with all the rigging details) includes a couple of merchant ketch rigs to work from.
As I said above though- the sharp lines (almost caricature) of the Pyro vessel might be difficult to disguise, certainly for a humble baltic ketch (Chapman calls this type by the Scandinavian name 'Galeas'- not to be confused with dutch Galleots, even though that was where their ancestry was laid).
As a rather bold suggestion however, Chapman includes a privateer ketch in is volume (a popular rig in C.18th Scandinavia) in plate XXIX (number 9) This would be an interesting model, and would allow the hull of thePyro bomb ketch to be used much more legitimately.
In this vein, why not have a search on the Danish maritime museum's website database of drawings? You might find something of use. here's a good ketch, which includes deck fittings (so rarely included in plans, and one the aspects of ships which Chapman rarely dealt with):
http://www.orlogsbasen.dk/billed.asp
Having looked through my copy, of Architectura Navalis Mercatoria, I notice from Chapman's rigging plan for his ketches that there are a few intersting features I hadnt notice before now- e.g- the mizen doesn't have a stay, just the foremost shrouds of the mizen drawn forward to be rigged to the mainmast channels. Interesting stuff!
hope this helps,
Will
edit- you may have noticed my avatar, which is a 'Yorkshire Billyboy', a ketch-rigged coastal trader of very full form that had direct lineage from the North sea ketches of the 18th century. The type was used pretty much unchanged right in to the last decades of commercial sail on the Humber coast of England.