faid you will battle to modify that amp purely because the circuitry is PCB based and is therefor not so modifiable.
If you like it so much replace the faulty crisps ?
I looked at the schematic briefly and there is precious little you can use from then entrails. my advice is turn it into an Marshal 18 watter,
The biggest cost of building amps and pedals goes to the hardware, chassis, enclosures, Speakers, transformers, fixtures and fittings. the lighter things are a few buck here and there. Most of this you already have
So lets see what you will need.
1 x Power Transformer - Karel can wind as can your local transformer winder
1 x Output transformer - Karel can wind as can your local transformer winder - Output tube specific
2 x 9 pin sockets and fittings - preamp and phase inverter
2 x power tube sockets 9 pin for el84 8pin for El34/6l6/
A turret board or termination boards
Tubes
a bundle of passive electronics ... and a handful of sockets .......this pic should give you an idea
http://www.trinityamps.com/ForumGallery/trinity/TrinityAmps18_Watt_Kit_sm.jpg
