I suggest to do something that makes sense as a function of your goals.
I understand that you are interested in neutron production on your lead-bismuth target. Then, neutron transport in the latter may not be a main issue, as I commented above at the end of my first post.
Define (properly, see above) your target isotope composition, such as the incoming proton will react on a target isotope correctly sampled by FLUKA. Assign to each mono-isotopic lead material (by means of the LOW-MAT card) the low energy neutron cross sections of the lead natural composition (this obviously is not correct, but it allows the produced neutrons to be transported below 20 MeV in the target). Score neutron double differential distributions by means of USRYIELD/EMERGING (see the manual and a fresh post). If you (really) need more, use mgdraw.f, but not the BXDRAW entry, rather the USDRAW entry (allowing to access the reaction products right after the reaction took place). Also, you may need to bias proton reactions by means of the LAM-BIAS/INEPRI card (in this case, statistical weights WEI (IP) become crucial in your USDRAW customized scoring, while they are automatically taken into account in the case of built-in USRYIELD scoring).