If you want a re build or it checked properly and rebuilt this chap (Andy Dawson) did mine for £80.00 including seals, oil, gas and a new bottom bearing.
Andy Dawson 07973 628270
He is the only person/shop I found that could do it. If you go elswhere and someone mentions drilling and fitting a schrader valve etc then run a mile!
Is the bike in reasonable nick? I doubt the spring could be knackered at that age and it should be ok for 65kg. I bet your gas is low.