Name's Onyx Statuette, member of the Directorate and subordinate of Keeper Revenant. I'll be discussing with you the technicalities of the engine-power core complex, modifications of it, and other formalities.
So, Osiris engine-power core complex. It's quite Libertonian in design: oversized for its efficiency and output, with a bulky turbine main chamber, elliptical power core housing, and redundant preburner and injector designs. "Charming".
Modifications of existing units are no easy task and while it's still less taxing on our manufacturing capabilities than building a new engine unit for capital class ship - it still takes resources to make the tweaks and time - to design it. The main point of 'miniaturization' will be replacing the power core unit, accomodating it with fitting 'preburners' and power lines, and cutting down the edges on the main turbine chamber of the engine unit, while modifying the main injection with additional performance packages to make up for the tighter turbine. Further adaptations of the nozzle design would be also beneficial. In short, refining the existing design, fitting it with a more efficient and smaller power core unit and other smaller changes to auxiliary systems would result in an 11.21% rise in efficiency while the overall size of the complex would be decreased to 68.33% of the original - it would be very tight in the engine bay yet it will fit into the cruiser-size hull. Closed-loop magnetohydrodynamic turbine designs of the Libertonian engineering doctrine will be always prone to failures, power surges, intermissive propulsion, turbine diaphragm breakdown, and sudden combustion chamber ruptures with combustible results when hit too hard - the only thing that can be done here is to add as many dampeners as possible and make sure your pilots don't ram things without the urgent need to do so.
I have the rough 'sketch' of the modified engine-core complex design transferring to you right now.
I would need some time to finish up this sketch and I still need to overcome some technical challenges - for example, how to use the existing integral 'preburner' unit in it but in general - this is what you will get.
On the topic of the other additions, such as sensor arrays and observational dome - I see no issue with just placing the antennae duplex on a moveable axis point and wiring it up to the power core unit and ship systems - you would need to turn the ship around and point the raised antennae array in the needed direction and in standby mode, it would be parallel to the ship hull. Observational dome - the question here is just where you want it, I see no issues in putting this anywhere where you see it would be fitting.
Contact me, will this work for you - and I'll try to finish up the design and I'll prepare the 'shopping list' for you. If Keeper won't say otherwise - you'll cover all costs.