It's worth experimenting with wrapping your existing pulley with something like Kapton tape. Use a strobe to check the speed and add as many layers as required to slow the platter down by the needed amount. Kapton is a good choice as it will lay even and grip the belt properly, whilst being thin enough to add a precise thickness.
Nottingham Analogue Spacedeck / Kuzma Stogi S / Benz Micro Ace S L
Lenco L-75 / Carl Ellis Plinth / Ortofon AS-212 / Ortofon SPU Classic GM E MkII
Ortofon ST-7 SUT / Nick G Modded Lentek Head Amp / Croft RIAA
Cambridge Audio CXU / Theta DS Pro Prime
Classé Model 30 / Luxman M-383
Usher Compass X-719
May contain opinions that others find disturbing.