If the end user was an authorised LH repair station with MGB capability then this O-ring would be available to them.
There is no alternate P/N listed in the CR&OP.......
Wings you are going down a very slippery slope here.....Even suggesting to look for just something that will work in place of the correct OEM P/N is inviting trouble. Just saying.
So who is removing the Input module from the MGB?