Good luck with the project Michael. When I first got into RS250s around the turn of this century bikes and parts were dirt cheap. How things have changed. I got 2 massive lots of spares including engines for very little money. If only prices had remained at a reasonable level.

And this....

https://www.ebay.co.uk/itm/256486015552?_skw=honda+cb250rs+engine&itmmeta=01JXPVH8BNAM37F39F7MV8V5JF&hash=item3bb7c20240:g:1eoAAOSwaj9mI6Uj&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1ehdnUXeQzs7C29NakcpfAGpvaeOaCHGGckJsIHUjvaPWONSKyzeOHrb0DAzADw570Zc7I2M5ZVzDem3torWqxBGyH5VCJsOuuRIk2cYxPpKAMZZoyssn%2FG0wzZhXGgZziPH%2BntXLmxmV1Jc5R6PK4gcUaqIwZtn6NkhVvaKJR%2FsGMECzVIqpEzlGuolZmh%2Br1xditCUlRp8TaHCtgIEZ1NaQei%2BUKjw3S%2BYoxZwnNOZHfVY1OfYqCpYrbdRIDgPktWHxAlT5Qx7gy2XyUfAlgoUhhBcFve3QraE88Nf1gCdw%3D%3D%7Ctkp%3ABk9SR-6HxdvtZQ

Is that engine even rebuildable?

Update : my fuel consumption figures were miles out! Not sure how I managed that? Any way corrected figures are 72 mpg for the first fill up and 81 for the second. Still not too bad but nowhere near my 500 Bullet which regularly returned over 100mpg.
Anyway I took the Guerrilla for its first service this week having just about clocked the 500 kms . No problems and it runs just the same after the service. I can now increase the load on the engine gradually up to about 1250 miles after which full power can be used.
The thing does want to go and begs to be revved but although I know running in's not likely to make much difference these days I still want to do it properly. I did 20 miles on it this morning after picking it up and this afternoon I took my 650 Interceptor on a 35 mile run into the Severn Valley. I wanted to get the mileage up to 7000 so I could do an oil and filter change. It's interesting to make a comparison as I've also owned the 650 since it was new in 2019. Obviously the 650 is a lot heavier and with the mods I've made (bigger bore exhaust and Norman Hyde cans + a DNA air filter kit) it's quite torquey now. Out of the box the 650 was always a bit lacklustre I thought but now it's quite pleasurable to ride. There's one single track  lane I've discovered this year this year out of Bewdley for anyone with local knowledge  which rises very steeply and then descends just as quickly down to a ford (fortunately the water at the moment runs under a bridge ) and then the road climbs steeply up again. There's lots of countryside with views and quite a few farms with machinery you have to watch out for as they never seem to slow down for other vehicles. The Guerrilla is more easily manageable with its lighter weight but at the moment I can't use all the performance which I don't think will be lacking once I am able to.

Nowt wrong with that piston that a good tig welder can't put right 😉.

Ahhh the joys of buying off eBay