The engine will physically fit in the 149. The frames in a wide frame (86-1x8/9) and the Quietlines (800-1x00/1x50) use almost the exact same frame. With a K341AQS in a 149 frame, you would basically have a 169....
But, you will have wiring work to make the Quietline engine work in the 149 frame. If you have the entire 1650, it may be easier to use the Quietline wiring harness in the 149. The 149 has a mechanical PTO Clutch, a Starter/Generator with the Voltage Regulator under the seat by the battery, the coil is on the carburetor side of the engine. The 1650 engine has an electric PTO Clutch (will need the wiring to support that), a gear drive starter that has a ring gear on the flywheel and the charging system is under the flywheel (I can't think of the technical name of it right now) and the voltage regulator is on the starter side of the engine, and the coil is on the starter side of the engine.... so the engine compartment electrical is different between the two series.
The 149 has the mechanical PTO lever on the side of the dash, and a 1650 has a PTO switch in the dash and a fuse for the PTO in the tower by the key and headlight switch. I've seen someone put a Quietline engine in a wide frame and rig the manual PTO lever to run the electrical for the PTO (it already has a safety switch on it, so a different switch could be used......)
As for running belt driven attachments, the wide frames and quietlines use the same belt size, and length. I've pulled a deck, mule drive and belt out from under a 149 and mounted it on on a Quietline. Moved all the tiller stuff back and forth (right now my 1450 has the tiller gear box from Dad's 149 on it and I use the same belt and mule drive). So if it worked with the 149 engine, it will work with the 1650 engine (we actually own multiple of each model 149, 1450, and 1650). Where switching belt driven attachments back and forth takes more work is between a Wide Frame/Quietline and an 82 Series. The 82 series uses a 5/8 belt and the mule drive pins are in a slightly different location. The WF/QL uses a 1/2 belt.