I can't recommend a replacement for your EMG81, but I was just wondering if the neck pickup's also an EMG?
You could have a problem mixing active and passive pickups on the same guitar, unless each pickup has separate volume & tone controls - the EMGs use 25K pots, but the passive pickups would need 500K.
If that's not an issue, I'm sure BKP could provide a cover for your neck pickup, but it would be best to get them to fit it for you so it can be potted - a loose cover can cause squeal and feedback. And there might be a problem of polepiece spacing not matching.