Hi body is a clip on affair - hold chassis and slide body back about 3mm and a locating lug on rear will unlatch and body can be rotated at front upwards and be lifted off, this works for all generation of bodies.
At this point to access the bogie gears which as you suggest are split you need to use a small screwdriver driver to lever off bogie sideframe. But each end differs. cab end bogie - at fuel tank enda gentle levering in the slot you can see will spring the plastic top piece over a lug in the bogie centre frame, watch how pickups are located behind part of this side frame.
Rear - pullout box that holds the coupler and again you can now see the slot- gentle lever of the bogie side frame -again watching pick up strips.
To test which wheelset is in trouble try to slide the gear on the wheelset axle sideways - a split one slides easily out of mesh. These can now side frame/keeper plate is removed be lifted out by pulling on one wheelet to release from bogie chassis unit until free and doing same on other side, watch how the pickup wiper contact rubs on wheelset - worth a clean of the wiper. New wheels from BR lines - specify for the HST as several diameters exsist.
It is not necessary to remove bogies from central chassis unit but there are two generations of chassis unit. The older has a black screw in lug that retains bogie - simply unscrew and remove lug to detach bogie.
At this point if it is the later "split frame" - they are painted black and as name suggest in two parts and are electrically split from each other, held in place by screws in plastic bushes and plastic nuts- all easily lost so take care. The bogies have a drive shaft and are retained in the frames by lugs into cutouts on the frame, the motor is held in place by the frames and the motor contacts are made to touch the sideframes to make the circuit. Some have a TV supressor on two little screws on top of chassis. They are easy to take apart, remove supressor if fitted and with chassis laid on side with screw heads looking up carefully undo all three and lift off upper chassis - easy but at this point it gets exciting as all components are now released. Note mark upward face of motor - if motor goes back in other way up the chassis will run in opposite direction to rest of the fleet.
You can extract bogies or change motor etc at this point reassembly is easy using three hands and luck, bad lagauge and 4 stirrers ! - the stirrers allow chassis to sit off work top a fraction and allow the wider bogie sideframes sit level on worktop, motor can be reinstalled and shafts fitted into bogie and slid into coupler flywheel unit. bushes fitted and top piece can be fitted back on - hopefully with screws in place and a bit of luck the bogies will rotate and tip up and down the screw tightess is important to allow bogies to rotate but also allow motor contacts to touch frames. Getting it right first time is not a given !