Welcome to Beoworld !
The Beogram 1100 uses a round (cross section) belt.
I can supply belts for most Beograms but I haven't yet found a producer capable of
making suitable belts for this Beogram series.
Size and friction is not the problem, the elasticity is.
The elasticity is particularly essential in these decks since the belt should of course
have a good grip around the platter and the idler wheel but at the same time it should also
allow the idler wheel to swing away from the motor pulley with only little force when changing speeds
and accept that the idler wheel sits farther away from the platter at higher speeds.
Without the correct elasticity, one or more of these functions won't work.
I've noticed one or two sellers on Ebay claiming to have suitable belts but
I haven't tried any of them myself, so I'm afraid, I cannot advice.
If everything else fails, ask B&O. They reproduced the round (cross section) belts a couple of years ago,
not cheap but I suppose they will work fine.
Martin