The engine fund should be there to cover a growing liability in the group accounts and is not an asset which is partly owned by the members.
Unless the group is dissolved in a scenario where the aircraft is scrapped, in which case all bank balances can be divided up between members.
The problem is that there are countless ways one can work out what the engine fund should be, and it will never be exact because engine overhaul costs vary so much according to where you go and what they find needs doing. One could be conservative and budget for a new engine (or some other known-cost option). But if you are conservative on this, you will have a higher hourly rate than other groups... and if you are not conservative then new members will end up subsidising the flying of previous members.
And that's just the engine fund. On the average 30 year old spamcan there is usually a constant stream of bits that need fixing, and somebody selling a share will know more about this than somebody buying a share
This is pretty significant; for example the engine fund might need topping up at £15/hour, but if the next Annual brings a suprise £5000 bill and the group did just 100 hours, that is another £50/hour. I suspect that on most old planes the normal maintenance cost well exceeds the engine fund.
There is no perfect way to do this.