As you may or may not know I have a habit of doing things with old boats!
Now, your bolts issue sounds a lot like the reason I now have a Merc on my transom.... I bought the boat with a Yam on the back, which suffered the same sort of thing as you. The rest you can piece together from my other posts!
If you don't have time to undo 4 bolts and disconnect a couple of throttle & gear cables to remove the engine form the boat then I would suggest you just stick it on & live with the loss. BUT... THe boat may be 60s, but the engine looks more like early 1980s to me. (one of the old 2 cyl premix jobs)
The time to get rid of a "package" (even a non runner) is Feb- May when people think it'll be a good wee project for this season". Can you live with it sitting there for 4-5 months? Put it in perspctive - I have found most of my "public sale" donor engines (or half engines) around this time of year, so for an hour or so to separate them.... (even if you are new to the game & it takes two half evenings - the engine is likely to be of interest to someone for the spares (esp. if it;s a working powerhead). You could then get a low HP working engine for £100 or less and sell the hull on as a "fishing punt" or "canal cruiser" - many will be put off by a dead engine.
Could you - Mail the Glastron Owner's club? someone might be willing to take the hull off your hands for the "fee" of removing the engine.... (which you then stick on your chosen mart site for spares)
The prop has to be worth £30 or so.
Now, being controversial, let's go back to your bolts.....
You say it's the impeller has gone. So presumably we are talking the bolts holding the gearbox to the leg? Could you (or your boat man) find a donor leg (off a seized powerhead) and combine the working bits? I bet that engine shared it's leg with a few other Hp Engines...... (My old Yam 55 of similar vintage I nearly got the leg off a 90 whilst trying to upgrade it to PTT!)
Or, buy a suitable sized engine, fit it, run it for a year and then punt it as a going concern. If your boat man deals in engines he will more then likely have a working trade-in suitable for that transom. I'll bet he charges as much or less to fit a working engine than faff with the one you have.
There's a place up here (Mtr Boatjumbes) bulk buys old trade- ins, gets the runners running & breaks the rest for spares. Granted not on your doorstep, but I got a working "classic" (i.e as old as me!) Aux off them a few months ago. he may have a leg or replacement engine.
Other famous places are Davie on here (totally reccomend). There's a place near the Humber bridge the name of which escapes me and the place that resurrected my 1972 Johnson with a new geartrain - Boating Mania in Upminster Then there are the Well known ones like and Bill Highman in Manchester..... and so the list goes on.
(Just be wary of one of your local "ebay shops" - see a long (#posts) long running (time) thread on here