The answer as regards the Valhalla is probably yes, but with several caveats.
Firstly, the Valhalla was in production for a while and there are many which are near end of life and will need some components replaced. Again, the components are available, but it does mean you wouldn't want to overpay for one. Some companies offer reconditioned ones, but they aren't necessarily cheap.
Secondly, although fitting isn't overly complex and the instructions can be downloaded from a number of sources, your crossbrace may not have the necessary holes drilled, which means either you strip down the deck and bodge a DIY job or get a more recent crossbrace, which effectively means the sub chassis.
Apparently the upgrade is well worth it in terms of speed stability. I have an older deck than yours and will be trying it at some point. You'll need the PCB clips which are cheap enough.
As to the motor, I'd stick with the Linn motor, if only for cost, plus they're (normally) rock solid.
You can use your existing pulley on the Promotec motor, but fitting is tight. You'll need to heat it up so it expands and fits on the shaft - they say boiling the pulley in water should be enough.
